[Read before the Wellington Philosophical Society, 5th April, 1906.]
On the 31st December, 1900, articles of contract were made by Her Majesty's Government, Canada, New South Wales, Victoria, New Zealand, and Queensland on the one part, and the Telegraph Construction and Maintenance Company on the other, for the construction and laying of the Pacific cable. The contract called for the completion of the whole cable on or before the 31st December, 1902. The cable was finished two months earlier, and, after undergoing the required test of a month, entered upon its commercial career on the 8th December, 1902. Thus was the project that had been advocated with persistence from some quarters for a quarter of a century an accomplished fact; the missing link of about eight thousand miles across the Pacific between Canada and Australia, in the world's metallic girdle, was now supplied.
Before laying a cable a survey is always made along the proposed route in order to select the most favourable ground, just as the railway engineer runs lines of levels before the final location of the railway. The cable engineer determines his levels by means of the sounding-line (piano wire), and at the same time obtains samples of the ocean-bed. It may be stated here that the direct route of the Pacific cable between the stations was departed from in order to avoid hills, craters, and hard or undesirable ground for the cable to rest upon.
From the survey the number of miles (nautical) required for the different flections was as follows: From Vancouver Island to Fanning Island, 3,654; from Fanning to Suva, Fiji, 2,181; from Suva to Norfolk Island, 1,019; from Norfolk to Southport, Queensland, 906; Norfolk to Doubtless Bay, New Zealand, 513.
The first section of the cable is about a thousand miles longer than any that had been laid before. This necessitated a considerable increase in copper for the conductor and in guttapercha for the dielectric. The working-speed of a submarine-telegraph cable depends on, and is inversely proportional to, the product of the total resistance of the conductor multiplied by the total electro-static capacity of the core, so that, other things being equal, the speed varies inversely as the square of the length of the cable. In the long section there were used 600 lb. of copper and 340 lb. of guttapercha per nautical mile; on the Suva-Fanning section 220 lb. of copper and 180 lb. of guttapercha; and on the remaining three sections the copper and dielectric were in equal proportions of 130 lb. each.
In the neighbourhood of Fiji, at a depth of 2,500 fathoms a temperature of 34.1μ Fahr. was noted, being the lowest temperature taken during the survey. There is very little difference in the temperature of the ocean at great depths, say below 3,000 fathoms, over a great extent of the earth's surface, the temperature being only a few degrees above the freezing-point, or 32μ Fahr.
The greatest depth, 3,070 fathoms (about three miles and a half), was found on the Fanning-Fiji section, where the bottom specimens consisted principally of radiolarian ooze. This ooze is found at the greatest depths, and was obtained by the “Challenger's” deepest sounding in 4,475 fathoms. The United States steamer “Nero” sounded in 5,269 fathoms (six miles), this being the deepest sounding recorded in the ocean, and the-material brought from the bottom was radiolarian ooze.
Of the 597 samples of sea-bottom obtained on the Pacific-cable survey, 497 were such that they could be divided into distinct types of deposits. It was found that 294 samples referred to globigerina ooze, sixty-five to red clay, forty-three to radio-
larian ooze, forty-five to coral mud or sand, twenty-seven to pteropod ooze, twelve to blue or green muds, and eleven to organic mud or clay.*
The pressure at a depth of 3,000 fathoms, in which a considerable portion of the Pacific cable is laid, is about 4 tons to the square inch. When the cable is being laid at such depths, it will be approximately twenty miles astern of the ship before it touches bottom.
Deep-sea cables last longer in the tropics than in the northern oceans. The reason is to be found in the fact that in the tropics marine life, from which globigerina ooze is derived, is more abundant than in the more northerly or southerly waters. It is the sun and the warmed surface-water that call into life these countless globigerina, which live for a short space, then die and fall to the bottom like dust, making such a good bed for the cable to rest in. In the arctic currents where the surface is cold the water does not teem with life in the same way as it does in the tropics, and consequently there is less deposit on the bottom of the ocean.
A submarine cable consists first of a core, which comprises the conductor, made of a strand of copper wires, or of a central heavy wire surrounded by copper strips as in the Pacific cable, and the insulating covering, generally made of guttapercha, occasionally of indiarubber, to prevent the escape of electricity. As far as cabling is concerned, this is really all that is necessary—an insulated conductor. This, however, would not, in the first place, be sufficiently heavy to lie in the ocean, and, secondly, would be too easily injured and destroyed by the many vicissitudes to which it would be subjected. For this reason a protection in the form of a sheathing of iron or steel wires surrounds the core, the nature, size, and weight of the sheathing being dependent upon the depth of the water and kind of ground over which it has to be laid. The deep-sea section, being the best-protected from all disturbing influences outside of displacement of the earth's crust by earthquakes or volcanic action, is naturally the one of the smallest dimensions; and for the shore end, which is exposed to the action of the waves; to driftwood, to the grinding of ice in the more northerly latitudes, and to the danger of anchorage, especially of fishing-boats, the sheathing must be very heavy. So that, while the deep-sea cable is somewhat leas than 1 in. in diameter, that for the shore end is nearly 2 ½ in. in diameter. The action of the waves is limited to a depth of only about 13 fathoms, so that their influence on the cable, manifested by wear and chafing, is confined to the shore end.
[Footnote] * Report of Sir John Murray.
The Pacific cable is equipped with the most modern apparatus at the various stations, and the cable is worked duplex—that is, messages are sent and received on the same cable at the same time.
Canada had carried longitude work from Greenwich across the Atlantic and thence to Vancouver. The completion of the British Pacific cable offered an opportunity for continuing the work across the Pacific in the interests of navigation and geography, besides tying for the first time longitudes brought eastward from Greenwich with those brought westward, making the first longitude girdle round the world.
In October, 1902, the Hon. Clifford Sifton, then Minister of the Interior, authorised the carrying-out of the transpacific longitudes, and the Governors of the South Seas, Australia, and New Zealand were respectively officially notified thereof. In preparing the programme for carrying out the work the climatic conditions of the various stations to be occupied were studied so that the most favourable times and seasons might be chosen. It was found that Suva, Fiji, was the governing factor, as it was by far the rainiest place of the series. The work was placed in my charge, and Mr. F. W. O. Werry, B.A., was associated with me as the other observer.
The instrumental outfit of the two observers was practically the same. Each observer was provided with a Cooke and Son astronomic portable transit, each of 3 in. clear aperture, the one of 34 in. the other of 36 in. focal length. Each transit was provided with reversing-apparatus. The transits of stars were observed over eleven threads in groups of three, five, and three respectively. The eye-piece attachment carried a micrometer (one revolution about a minute of arc with thread parallel to the transit threads) for latitude work; and the whole attachment was necessarily movable through 90μ, so that the movable or micrometer thread becomes horizontal. The recording of transits was made, by means of a key, on a Fauth barrel chronograph. Each observer was provided with two sidereal box chronometers, one being a spare instrument in case of accident. There were, besides, dry cells, switchboards, and minor accessories to complete the outfit. I carried, too, a half-seconds pendulum apparatus and a Tesdorpf magnetic instrument, the latter similar to the ones furnished Drygalski, of the “Gauss,” on his Antarctic expedition.
At each station—that is, at Fanning, Suva, Norfolk, South-port, and Doubtless Bay—a brick or cement pier was built, and an observing-hut covering the same. At Vancouver, which is used as a longitude reference point for the whole of British Columbia, we have a permanent transit-house.
Bamfield, on the west shore of Vancouver Island, is the eastern end of the Pacific cable, and was not occupied as an astronomic station, but simply as an exchange station—that is, for the comparison of the Fanning and Vancouver chronometers, to be described more fully later.
Longitude work consists in simply determining the accurate sidereal time for each of two places, the longitude of one of them being known, at an absolute instant, and then comparing such times: the difference between them will be the difference in longitude. The operation may be briefly stated: Each observer determines the error of his sidereal chronometer at a particular instant; then by means of the telegraph line or cable the two chronometers are compared, to be explained later; this comparison may be likened to an instantaneous photograph of both chronometers. Applying the respective chronometer corrections for the instant of comparison to the times thus shown by the two chronometers, we obtain the absolute local sidereal time for each place for the same instant; and, as before, the difference between these times is the difference of longitude.
Now, suppose we have a transit instrument with a single vertical thread, and that thread situate in the axis of collimation; furthermore, the axis of the telescope horizontal, no inequality nor ellipticity of pivots, and the pointing of the telescope truly in the meridian; then, if we record the transit of a star across the thread, and the time noted is free from personal equation, we obtain immediately the clock-corrections by comparing the observed time with the right ascension of the star for that time and day. The many conditions imposed in the last sentence show the many sources of error, the effect of which must be evaluated ere we obtain the desired quantity—the clock-correction; in other words, the true local sidereal time at a given instant.
We must therefore devise means for determining the instrumental errors, some of which are practically constant—inequality and ellipticity of pivots; while the others—level, azimuth, and collimation—are more or less variable from day to day. Careful readings, at the beginning and end of a season, of the former will evaluate them. For the latter we will speak of the level-corrections first. This quantity is determined directly by means of the striding-level placed upon the axis of the instrument. Readings should be taken as frequently as the intervals between stars admit. With sensitive levels, reading about a second of arc for divisions, great care must be exercised in allowing the level to come to rest. My own practice is not to take a reading until fully a minute has elapsed after placing the level, and as a light is necessary for reading at night, the reading should be taken quickly, for even a short exposure of the level to light
will cause a change in the reading. I consider a six-minute interval between stars the minimum during which a deliberate reading (including reversal of level) for inclinations of the axis can be made. How to treat the various level-readings for one position of the instrument will depend upon circumstances. The readings may show a decided and unquestionable gradual change of level; in such a case the readings may be plotted and the level-reading for each star interpolated therefrom. If, on the other hand, the level - readings are confined within the errors of reading and small fluctuations, we may then take the mean of the various readings as the reading for that position of the instrument. The angular value of the level - reading expresses the angle between the vertical plane (in the case under consideration the meridian) and that described by the transit; the two great circles intersect each other in the horizon, where the level-correction is nil. The level factor, usually designated by B, is expressed by cos (φ—δ) sec δ. This factor computed for each star, multiplied by the inclination of the axis, expressed in time, gives then the level-correction to be applied to the respective transits. Errors of level are measured directly, while those of azimuth and collimation with portable astronomic instruments are not directly measured, as is the case with the large transits in observatories. This leaves then the determination of three unknowns—the azimuth, collimation, and clock corrections; the minimum number of stars to determine which is three. With only three stars, however, there would he no measure of the accuracy of the observations, for one, and only one, value for each of the unknowns would satisfy the three observation equations; there would be no probable error. If the instrument is not in the meridian it is evident that the times of transit of stars north of the zenith will suffer a correction of opposite sign from those to the south. If the telescope is pointing west of north, north stars transit too late, and south stars too soon; and vice versa if pointing east of north. As polar stars move slowly they are well adapted for obtaining the azimuth-correction, and hence one polar star is included in each time set for each position of the instrument, and the general azimuth-factor is sin (φ—δ) sec μ.
With the collimation-error, however, the correction for north and south stars is of the same sign for one position of the instrument; but when the instrument is reversed, then the error is of opposite sign, and the transits of stars are similarly affected. The effect of the collimation - error becomes therefore more apparent and is more accurately deduced when some stars are observed in one position of the transit, and others with the telescope or axis reversed.
The effect of the collimation-error on the times of transit varies directly as the secant of the declination of the star, hence the collimation factor is sec μ.
In order, therefore, to obtain a satisfactory time-determination—which is really the quantity sought—we observe more than the absolutely necessary three stars, and find the most probable value by the method of least squares.
In the programme of the transpacific longitudes it was arranged that (barring cloudy nights) on each night there should be two independent time - determinations; each determination to be derived from fourteen stars, divided into two groups of seven each, of which one was a polar. Furthermore, one group was observed clamp east, and the other clamp west. The six other stars of each group were “time” stars, and selected near the zenith and south (in the Northern Hemisphere) thereof. Instead of three we now have fourteen observation equations from which to deduce the three unknowns, already mentioned, by the usual method of forming the three normal equations. It is desirable to reduce the effect of azimuth and collimation on the derived clock-correction; we attain this by making the algebraic sum of the azimuth-factor as small as possible, and similarly with the algebraic sum of the collimation-factors.
In deducing the time - correction it evidently must signify the correction at some particular epoch, for every clock and chronometer has a rate. The epoch chosen is generally the mean of the various transits constituting a set, and the transit of each star is corrected for rate, as if all stars had been observed at that mean time. If, after having obtained the azimuth and collimation errors, we apply them with their respective factors to each transit and compare this corrected transit with the apparent right ascension corrected for aberration, we obtain the clock-correction of that transit or star, and the difference between this and the clock-correction of the normal equation gives us a residual. Each star thus furnishes a residual, and from them is found the probable error of a single observation as well as of the deduced clock-correction from all the stars. The average probable error of the latter is about 0.01 s. for good work.
A word about rate. Rate is one of the most difficult problems with which we have to deal in field longitude work. It is not the magnitude of the rate, although a small rate is very desirable, but the constancy: this is the crux. A chronometer may have an apparently constant daily rate, yet the hourly rate for the twenty-four hours may and does vary. Again, the rate is not the same when the current is on as when it is off; the former obtaining when observing and the latter the rest of
the day. The rate deduced from two independent time-determinations of the same night, when the temperature is practically constant during the time of observation and the clock is in circuit with the battery (one cell) only during that time, is seldom, if ever, the same as that obtained from day-to-day observations.
In our programme we have two independent time-determinations for each night. Each set of transits is reduced to the epoch of the mean of the times of transit of the stars comprising that set. The rate which is applied for each transit to the mean epoch, and for which some magnitude must be assumed, is practically a vanishing quantity in the resulting clock-correction. The ideal time of exchange would be at that epoch when the effect of rate is eliminated. But, for various reasons, this is found to be impracticable. In the programme, then, of two independent time-determinations, for obvious reasons the exchange was arranged to take place about midway between the two epochs.
An interpolation between the two epochs gives the clock-correction at the instant required—that of the signals. This assumes that the rate is constant during the interval and is represented by a straight line. If extrapolation is necessary, as sometimes occurs, the rate-value has less weight. It is highly desirable that the temperature of the chronometer be kept as uniform as possible, and, if necessary, special provision made to attain this end.
We are supposed now to have made a complete time-determination, and are ready for exchange of signals—that is, of a comparison between the two clocks of the two stations.
As some of the exchanges were over land lines, I shall explain this method of exchange first, taking the case of Vancouver and Bamfield. Each of these stations was supplied with a switchboard. The portable switchboard has been in use many years and has given every satisfaction. On it are mounted a talking relay, a signal relay, and a pony or clock relay; the last is never on any circuit but that of the chronometer with one dry cell. Besides, there is an ordinary talking-key and a signal-key, the latter breaking circuit when depressed while the ordinary telegraph-key makes circuit. Along one edge of the board there is a row of binding-posts for connecting with the clock, chronograph, main line, and batteries, of which there are three dry cells for the chronograph, and, as stated, one for the chronometer. And, lastly, there is a three-point switch, by means of which the main line can be thrown on or off the points of the clock relay, and plugs to cut in or off any relay. While observing, the chronograph-circuit passes over the points of the clock
relay, and, as the clock or chronometer breaks circuit every two seconds (omitting the 58th second so as to identify the minute), the points of the clock relay separate every two seconds, and hence record the clock-beats on the chronograph. In the chronograph-circuit is the break-circuit observing-key too, by means of which the transit of each star over the eleven threads is recorded.
It is customary when beginning the exchange to put the telegraph-line for a minute at each station over the points of the clock relay, whereby the circuit of the main line is broken by each chronometer every two seconds—that is, we let the clocks (chronometers) record simultaneously over the line, each chronograph thus obtaining the record of both clocks. From this record we immediately see the relative position of the respective minutes—in fact, of the seconds too—enabling one readily to identify corresponding arbitrary signals, by means of which the more accurate chronometer-comparison is made. Theoretically, the comparison by the chronometers recording directly over the line, as above, is as good as by arbitrary signals. The trouble lies in scaling or measuring the former. As, for an interval of a minute, the relative position of the two-second breaks of the two chronometers is the same, after having measured one such interval on the chronograph sheet the mind is involuntarily biassed; we know that all the others should be the same, and, consequently, we cannot measure, say, thirty, our minimum number, with that freedom of mind which would be the case if we did not know what measure to expect: hence the device of the arbitrary signals. In this case each chronometer records only on its own chronograph. One observer now sends by means of the signal (break-circuit) key twenty arbitrary signals; the chronograph - circuit, which always passes over the points of the clock relay, is now made to pass too over the points of the signal relay, which is on the main-line circuit. Hence a signal sent will be recorded on each chronograph, and each chronograph has its own chronometer-record for interpreting any signal, just as it interprets the transits while observing.
As the word implies, these arbitrary signals are intentionally made irregular, and will average about two seconds apart. The other observer now sends, similarly, forty signals, and again the former twenty more, so that the mean of the times of sending of the two observers about coincides, thereby eliminating differential rate of the two chronometers. It is customary when sending signals to give a rattle with the key at the beginning and end of each set. If there is no trouble on the line the whole exchange is over in five minutes. A few minutes are required for conversation about the condition of the sky. If the prospects
are hopeless for the night for one, the other desists from further observations. The accuracy with which these comparisons are made is far beyond the accuracy that is possible in a time-determination: while the probable error of the latter is, say, 0.01 s., that of the former is generally less than 0.002 s.
The exchange on the cable is similar to that just described of arbitrary signals. The chronograph here is replaced by the paper fillet of the cable service. It is scarcely necessary to observe that nowadays signals (messages) on the cable are not read by means of deflections of a small mirror, interpreted on an opal glass scale by means of a reflected beam of light, but are read from the fillet of paper on which a siphon records in ink the deflections. As the current is very weak the siphon is not in direct contact with the paper, but, by an ingenious vibrating device, it deposits a tiny drop of ink at very brief intervals. A cable message looks like a profile of the Rocky Mountains, the ups and downs having an interpretation like the dots and dashes in the Morse system of telegraphy. From experience it is found impracticable to have the clock recording directly on the cable for interpreting signals sent or received. However, it is necessary to have a time-measuring scale on the fillet. We accomplish this by attaching another siphon to the frame of the cable instrument. This one is quite independent of the cable. It is actuated by a long vertical rod attached to the horizontal arm of an ordinary sounder, and connected to the siphon by a silk fibre. This latter siphon drags an ink-line on the fillet. The sounder is put in circuit with the clock, and hence every time the clock or chronometer breaks circuit the sounder makes a sharp break in the line on the fillet, and a time-scale is obtained close to and parallel to the zero-line of the cable-siphon. By projecting vertically these recorded clock-breaks on to the cable-siphon record, we can interpret in time the arrival or departure of a signal. We must know, however, the relative position of the two siphons. The signals are sent with one of the two cable-keys (on cables there are always two keys, one for sending positive and the other for sending negative currents). To the lever of the cable is adjusted another lever which is in the clock-circuit. It is so adjusted that the moment the cable-key makes contact—that is, sends a current into the cables—at the same moment the clock-circuit is broken, thereby both siphons record the event simultaneously, and the parallax between the two siphons is obtained. As a check on the value thus obtained for the parallax, a slight tap is given to the frame carrying both siphons, thereby disturbing both, and the parallax obtained. By the above arrangement, when sending signals we have two records on the fillet, one by the clock-siphon, the other by the
cable-siphon. In receiving signals there is, of course, only the record of the cable-siphon, the other siphon recording only the chronometer-beats, which, on the fillet, measure about 1 in. for the two seconds. The speed of the fillet may be varied to any degree. It will be seen that a comparison of clocks by this means is simply a matter of careful linear measurement. Were the records at the two stations instantaneous, then the two records would be identical; but such is not the case. Each signal arrives late at the distant station, and therefore the two records will differ by twice the time of transmission, assuming that the time of transmission is the same in each direction, an assumption which we cannot avoid. On the long section of the cable between Bamfield and Fanning, about four thousand two hundred statute miles, the time of transmission was a third of a second, equivalent to about twelve thousand statute miles per second.
In the first longitude-work by cable before the introduction of the recording-siphon, instead of arbitrary signals, the clock-beats were sent by hand at intervals generally of ten seconds, and the time of arrival of the signal, as indicated by the reflecting-galvanometer, was noted by the “eye and ear” method. The uncertainties and “personal equation” in this method of exchange and comparison of clocks are apparent.
We have now explained briefly how the clock-correction is obtained for a given instant, and how the comparison of the two clocks is made. The application of the clock-corrections respectively to the times of exchange gives apparently the local sidereal time for each place at the same instant. Each value is, however, affected by a small correction—the personal equation of each observer. As the quantity sought is the difference between the local sidereal times, the absolute personal equation of each observer is unimportant; it is the difference between the two personal equations that affects the difference of longitude. On land lines, where the ready means of transportation is good, it has been customary (up to the present, when, by the introduction of the registering-micrometer, the personal equation is eliminated) for the observers to exchange stations, the mean result of the two differences of longitude being free from personal equation: this is on the assumption that the personal equation of the observers remains constant during the longitude campaign. On this assumption, if there is a series of stations odd in number, and the observers occupy alternate stations, it will be seen that the odd-numbered stations will be free from personal equation, and the even-numbered ones affected by it. Now, between British Columbia and Australia, and also between British Columbia and New Zealand, the number of stations is odd—i.e., there are three intermediate stations, Fanning, Suva, and
Norfolk; hence Southport (Queensland), Doubtless Bay (New Zealand) and Suva (Fiji) are free from personal equation.
Personal - equation observations were, however, made at Ottawa by the two observers using the same clock and determining its correction at the same time on the same stars with the two transit instruments, and the resulting difference of personal equation, 0.124 s., applied to Fanning and Norfolk.
Southport was connected with the observatories at Sydney and at Brisbane, and similarly Doubtless Bay with the observatory at Wellington. Personal-equation observations were made between the respective observers.
It was on the 29th September, 1903, that the first satisfactory clock exchange was had with Sydney, and so this night may be considered as the one when for the first time longitude from the west clasped hands with longitude from the east, and the first astronomic girdle of the world was completed. The immediate reasons for the first telegraphic connection in longitude between Australia and the prime meridian, Greenwich, were (1) with a view of confirming the position of the eastern boundary of the Colony (now State) of South Australia, 141° E.; (2) for obtaining the longitude of stations to be occupied for observing the transit of Venus in 1882. To attain this end connection was made astronomically between Sydney, Melbourne, Adelaide, Port Darwin, and Singapore. A connection was made, too, between Sydney and Wellington. All Australian and New Zealand longitudes at present rest on the position of Singapore as accepted in 1883, which then, quoting from the Government report for 1886 of South Australia, “had twice been telegraphically determined—first in 1871 by Dr. Oudeman, of Batavia, and Mr. Pogson, of Madras, and more recently by Commander Green, United States Hydrographic Department.” The determinations of the latter were accepted. It may be remarked that at this time the Thomson (Lord Kelvin) recording-siphon had not yet been introduced, and that the clock exchanges between Port Darwin and Singapore over the cable were made by use of the deflecting mirror or reflecting galvanometer, already spoken of, a method involving more or less uncertainty in noting by “eye and ear” the movement of the mirror and the instant of time of its occurrence.
Singapore was dependent in position upon Madras, the initial meridian for the great trigonometrical survey of India.
For over a century observations have been taken from time to time to determine the longitude of Madras. The early ones, before the advent of cables and telegraphs, were dependent mostly on lunar observations, some on Jupiter's satellites. In 1891 the Survey of India had not adopted the then best value, so that at the International Geographic Congress held at Berne
in that year the question arose, why the known error in longitude of 2′ 30″ was not corrected on the Indian maps and charts. This gave rise to a discussion in India, and the whole longitude work was reviewed, with the result that a determination de novo was decided upon, carrying the work directly from Greenwich via Potsdam, Teheran, Bushire, and Karachi, where connection was made with the three arcs of the great trigonometrical survey between Karachi and Madras. This work was carried out by Captain (now Major) S. G. Burrard, R.E., and Lieutenant Lenox Conyngham, R.E., in 1894-6. The resulting longitude of Madras was 5 h. 20 m. 59.137 s. ± 0.022 s.
In 1903 a redetermination of Greenwich-Potsdam was carried out by Professor Dr. Albrecht and Mr. Wanach. Stations were exchanged and observations made with a Repsold registering-micrometer. The exchange of stations was made to test the elimination of personal equation by means of the registering-micrometer, and the result was highly satisfactory, the weighted mean of the one result agreeing with the weighted mean of the other to the third place of decimal of a second of time. It may be stated here that the introduction of the registering-micrometer in longitude-work marks a distinct epoch in that class of work, not only in assuring greater accuracy in the results, but also in very materially reducing the cost of longitude-work of the first order by saving of time and money in doing away with the necessity of exchange of stations. Since the completion of the transpacific longitude-work, the two Cooke transits used in that campaign have been provided with the registering-micrometer made by Saegmueller, of Washington, and the longitude work of 1905 was carried out with that attachment.
From the 1903 determination by Albrecht we have for the longitude of Potsdam 0 h. 52 m. 16.051 s. ± 0.003 s. This value is 0.098 s. greater than that of Burrard obtained in the series of 1894-6 referred to above.
In the reduction (1885) of the Australian longitudes, the longitude of Madras was accepted as 5 h. 20 m. 59.42 s., and the derived value of Sydney was 10 h. 4 m. 49.54 s.
In making the comparison between the longitude of Sydney as brought from Greenwich eastward with that brought westward, the best and most recent available data are utilised for the longitude of Madras.
Taking, then, Albrecht's value for the arc Greenwich-Potsdam, and the values of Burrard for the arcs Potsdam-Madras, we obtain for the longitude of Madras 5 h. 20 m. 59.235 s. ± 0.021 s.
As there have been no new determinations of the various arcs from Madras to Sydney, the values given in the report of May, 1885, by Ellery, Todd, and Russell, on Australian longitude, are used. Adding the latter to the above-accepted value,
we obtain for Sydney 10 h. 4m. 49.355 s. ± 0.088 s. The Canadian value is 10 h. 4 m. 49.287 s. ± 0.058 s. Difference, 0.068 a. = 1.02″ = 84 ft. for the latitude of Sydney—that is, the first girdle of the world closed within 84 ft.