Papers.—1. “On the Importance of Tide-gauges, and Description of a Simple Tide-gauge, invented by the Author,” by Sir James Hector.
This paper gave reasons for accuracy in our definition and records of what is known as the “sea-level,” instancing its bearing on the value of property, as it is the datum line from which all land surveys are necessarily made; on the coastal navigation of the country, as any permanent changes in the level would affect the depth of water on shoals and sunken rocks; on the orographical features of the country, as it was the datum-line to which all altitudes used by engineers were referred. Yet the sea-level was exceedingly variable. The chief variation was, of course, the well-known, but still imperfectly understood, diurnal tide caused by the influence of the Moon's attraction modified by that of the Sun, and to some extent also by the planetary influences. Theoretically, this great wave should sweep evenly round the globe, but, practically, it was first generated in the Southern Hemisphere, and thence travelled into the intricate land-locked oceans of the Northern Hemisphere. This at once indicated the important influence of the form of land and depth of oceans upon such waves. A variation in atmospheric pressure also affected the sea-level, and especially when it led to the propagation of surface-waves’ by violent storms or prevailing. winds. Changes of sea-level were also due to vulcanicity, which caused an absolute change by shrinkage or expansion of a portion of the Earth's crust; and, lastly, in the case of earthquakes, (which were impulses induced by explosive action below the Earth's surface,) waves, though less transient in their effect, disturbed the sea-level on a much more gigantic scale than any of the preceding causes.
Tide records and barographs were to the physicist as the pulse was to the physician. Experience had shown that while the mercurial barometer and elaborate tide-gauges were essential to refined and accurate measurements, even a rough apparatus, if sufficiently inexpensive to allow of its being used at many stations, would give more valuable results for general purposes. He instanced the data obtained from tide-gauges at the time of the Krakatoa eruption; but these were not thoroughly reliable, on account of our not having sufficient points of observation to determine how such ocean waves as then spread over the globe were influenced by the form of land past which they swept, or by the tide-waves, and still more by the tidal currents set up in the narrow straits, which they encountered. A few years ago tide-gauges were set up at Lyttelton and Dunedin, and within the last few months one had been erected by the Harbour Board in Wellington. All these gauges were of the best and simplest forms at present known; but
he pointed to the diagram of the Lyttelton gauge, and showed how it failed at the critical moment at the time of the Krakatoa disturbance, when the sea-level receded to at least three feet below the lowest water previously recorded. What was wanted, therefore, was a cheap simple tide-recorder, and plenty of them, at every lighthouse and every suitable point round the coast; and the result would be the accumulation of records that would have extreme value for future reference.
After much consideration, he had arrived at the form now exhibited, which was an adaptation of compound diagonal levers that moved vertically above and below a fixed point, the difference in the number of parallelograms above and below this point determining the degree to which the scale of motion is reduced, by an automatic pen marking a diagram on paper carried forward by clockwork. The model and drawings were then fully explained, and the hope expressed that before long this instrument or a similar one would be extensively used.