TRANSACTIONS OP THE SECTIONS. 9 



by aid of a microscope to observe the rise and fall of the water in the smaller cistern 

 to -i}ff of a millimetre ; but no such quiescence of the atmosphere could be expected 

 at any time ; and it is probable that the variations of the water-level due to diflerence 

 of the barometric pressure at the two" ends would, in all ordinary weather, quite 

 overpower the small effect of the lunar tide-generating- motive. If, however, the 

 two cisterns, instead of being open to the atmosphere, were connected air-tightly 

 by a return-pipe with no water in it, it is probable that the observation might be 

 successfully made : but Sieniens's level or some other apparatus on a similarly small 

 scale would probably be preferable to any elaborate method of obtaining the result 

 by aid of verj' long pipes laid in the ground ; and I have only called your attention 

 to such an ideal method as leading up to the natural phenomenon of tides. 



Tides in an open canal or lake of 12 kilometres length would be of just the 

 amount which we have estimated for the cisterns connected by submerged pipe : 

 but would be enormously more disturbed by wind and variations of atmospheric 

 pressure. A canal or lake of 240 kilometres length in a proper direction and in a 

 suitable locality would give but 10 millimetres rise and fall at each end, an efl'ect 

 which might probably be .analyzed out of the much greater disturbance produced 

 by wind and ditlerences of barometric pressure ; but no open liquid level short 

 of the vngens cpqitor, the ocean, will probably be found so well adapted as it 

 for measuring the absolute value of the disturbance produced on terrestrial 

 gravity by the lunar and solar tide-generating motive. But observations of the 

 diurnal and semidiurnal tides in the ocean do not (as they would on smaller 

 and quicker levels) suffice for this purpose, because their amounts difter enor- 

 mously from the equilibrium-values on account of the smallness of their periods 

 in comparison with the periods of any of the grave enough modes of fi-ee 

 vibration of the ocean as a whole. On the other hand, the lunar fort- 

 nightly declinational and the lunar monthly elliptic and the solar semiannual 

 and annual elliptic tides have their periods so long that their amounts 

 must certainly be very approximately equal to the equilibrium-values. But there 

 are large annual and semiannual changes of sea-level, probably both diflerential 

 (on account of wind and differences of barometric pressm-e and differences of 

 temperature of the water) and absolute, depending on rainfall and the melting 

 away of snow and return evaporation, which altogether swamp the small semi- 

 annual and .inuual tides due to the sun's attraction. Happily, however, for our 

 object there is no meteorological or other disturbing cause which produces periodic 

 changes of sea-level in either the fortnightly declinational or the monthly elliptic 

 period ; and the lunar gravitational tides in these periods are therefore to be care- 

 fully investigated in order that we may obtain the answer to the interesting ques- 

 tion, how much does the earth as an elastic spheroid yield to the tide-generating 

 influence of sun or moon.' Hitherto in the British-Association Committee's 

 reductions of Tidal Observations we have not succeeded in obtaining any trust- 

 worthy indications of either of these tides. The St.-George's pier landing-stage 

 pontoon, unhappily chosen for the Liverpool tide-gauge, cannot be trusted for such 

 a delicate investigation : the available funds for calculation were expended before 

 the long-period tides for Ililbre Island could be attacked, and three years of Ivur- 

 rachee gave our only approach to a result. Comparisons of this with an indication 

 of a result of calculations on West Hartlepool tides, conducted with the assist- 

 ance of a grant from the Royal Society, seem to show possibly no sensible yield- 

 ing, or perliaps more probably some degree of yielding, of the earth's figm-e. The 

 absence from all the results of any indication of a 186 jearly tide (according to 

 the same law as the other long-period tides) is not easily explained without 

 assuming or admitting a considerable degree of yielding. 



Closely connected with the question of the earth's rigidity, and of as great scien- 

 tific interest and of even gi-eater practical moment, is the question, llow nearly 

 accurate is the earth as a timekeeper ? and another of, at all e\ents, equal scientific 

 interest. How about the permanence of the earth's axis of rotation ? 



Peters and Maxwell, about 35 and 25 years ago, separately raised the question, 

 How much does the eai-th's axis of rotation debate from being a principal .axis of 

 inertia ? and pointed out that an answer to this question is to be obtained by 

 looking for a variation in latitude of any or everv place on the earth's surface in u 



1870. ■ 2 



