The author explained that total or annular eclipses could only take place when the line of syzigies coincided with the line of nodes, and partial eclipses of the sun and moon when their angular distances did not exceed 12° and 15° respectively; that since nineteen synodical revolutions of the nodes occupy 6585.78 days, and 223 revolutions of the moon, 6585.32, the relative positions of the bodies are brought back at the end of that period of 18 years and 11 days to the same position they occupied at the commencement of it, except the small amount of motion due to the 11 hours of difference, and apart from the effects of perturbations, and that all the eclipses recur under nearly the same circumstances as before. The knowledge of this period, and of the order of previous eclipses, as seen from one place, enabled the priests in the dawn of
history roughly to predict the future ones. Owing to the complexity of the moon's motions, approximate truth only was obtainable, even down to the eighteenth century, and the computation was made by the circuitous method of the nonagesimal. In our day, owing to the perfection of the tables published in the “Nautical Almanac,” the computation of eclipses or of occultations of stars by the moon is rendered very easy.
These phenomena are predicted in the “Nautical Almanac” generally from the centre of the earth, and their visibility is stated within certain parallels of latitude. But, from the effects of parallax, these times may vary by two hours either way for a place on the earth's surface, or the occultation may not occur at all in some positions within the limiting parallels.
An independent prediction is therefore required for every place; this may be made approximately by a graphical projection of the figures of the bodies in their true proportions and relative positions, which requires only a pair of compasses and a scale of chords, such as is to be found on a foot rule, so as to show the times within a very few minutes. If accuracy is required, the true relative positions of the two bodies can then be taken out for the assumed times, and the error to be applied to these assumptions ascertained by simple computations in spherical and plane trigonometry.
In our day the mere occurrence of the phenomena at the exact times predicted for them has lost the interest it had for our forefathers, to whom it appeared wonderful and mysterious; but a new source of interest in total eclipses of the sun has arisen, from the means which the spectroscope and polariscope afford for investigating the curious phenomena of the corona, red flames and sierra, and the means which photography supplies, with the aid of a suitable telescope, equatorially mounted with clock motion to keep the object steadily in the camera, of taking accurate pictures of the momentary appearances.
The various hypotheses to account for these appearances have now subsided into a clear knowledge that the sun is surrounded to a depth of two or three seconds of arc, corresponding to a depth of 900 to 1,200 miles, by an atmosphere chiefly of hydrogen, but mixed with the incandescent vapours of many other metals in a most tumultuous condition; that prominences, consisting essentially of hydrogen heated far beyond any temperature obtainable on this earth, shoot out from it to a height of 90,000 or 100,000 miles in very short intervals of time, and that outside that to an indefinite distance extends a radiated luminous envelope, which cannot be considered material, but which gives spectra strikingly similar to those of the Aurora and of the zodiacal light; that these effects seem to be associated with electrical discharges, and may hereafter prove a visible link between radiated heat, light, and electricity, the intimate connection of which is shown in many ways.
Some remarks were made upon the apparently small density of the vapours near the sun under so prodigious a vertical pressure.
The author then referred to the projected expedition to Cape York to witness the total eclipse of 12th December, and expressed a trust that it would furnish a quota to the scientific information derived from the many expeditions sent by the Governments of Europe and America to many parts of the world to witness the eclipses of 1868, 1869, and 1870, which would be worthy of the young energies and rising greatness of Australia.
This paper was copiously illustrated by large diagrams.
5. Mr. Dyson suggested the desirability of procuring drawings and specifications, or, if practicable, working models of the machinery used in various manufactures, for display in the Museum. He believed that an exhibition of this kind would be highly attractive to the public, and of direct practical value to those about to embark in manufacturing pursuits.
Mr. Tinne supported the suggestion, and stated that the Superintendent was engaged in obtaining information relative to several manufactures which it was desirable to introduce into the colony.
The President considered it doubtful if manufacturers of machinery would care to forward drawings and specifications of their respective machines simply for exhibition, although there could be no doubt of the actual value of information on the subject. He ventured to suggest to members the desirability of making themselves acquainted with the new adaptations of force to industrial pursuits which were continually being reported in the scientific periodicals of the day, and instanced the adoption of nitro-glycerine for blasting purposes in the Welsh slate quarries as being well worthy of careful study by all who were interested in the Thames goldfields.