Wellington Philosophical Society. 443 



7. Live specimens were exhibited of Helix busbyi, the large 

 land shell, collected by Mr. Munce, at Hokianga, and also 

 another live pair of the allied species of H. hochstetteri, recently 

 obtained near Collingwood, in the South Island. 



8. Photographs of native carvings and monuments in Easter 

 Island, also of a map, presented by Mr. B. Biss, were exhibited. 



Fourth Meeting: 24f/( August, 1887. 

 Dr. Hutchinson, President, in the chair. 



New Member.— -Mr. T. Wakelin, B.A. 



Papers. — 1. " On the Importance of Tide-gauges, and Des- 

 cription of a Simple Tide-gauge, invented by the Author," 

 by Sir James Hector. 



Abstract. 



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 exceed- 

 ingly 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 measure- 

 ments, 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 



