DEDUCTIONS FROM THE TIDES. 23 



and this would reduce the amount of fluidal movement on which a mass- 

 tide depends. It is not unlikely, therefore, that some part of the scantiness 

 of the atmospheric tide is due to the elastic constitution of the atmosphere. 

 There is a semi-diurnal wave of atmospheric pressure which has its 

 maximum about 10 o'clock a.m. and p.m. Lord Kelvin, interpreting 

 this as an increase of mass corresponding to the increase of pressure, 

 has computed that it would accelerate the rotation of the earth about 

 27 seconds per century.^ If however this oscillation is merely a transient 

 increase of elastic pressure at the base of the atmosphere due to basal 

 heat, the expansional effects of which are resisted for the time by the inertia 

 of the air above, as seems not impossible, the wave would have no direct 

 accelerative effects on the earth's rotation. 



THE TIDES OF THE LITHOSPHERE. 



There is reason to suspect that the water-tides are in part derived from 

 the pulsations of the lithosphere.^ It will therefore be best to discuss 

 these first. Since no body is absolutely rigid, and since abundant evidence 

 shows that the lithosphere is appreciably yielding, there can be no theo- 

 retical doubt that there are tides of the lithosphere of some kind and of 

 some magnitude. The only vital questions therefore relate to their magni- 

 tudes and their specific forms. 



The experimental efforts of Sir George and Horace Darwin,' of Von 

 Rebuer-Paschwitz,* and of Ehlert,^ resulted in detecting only slight indica- 

 tions of body tides, and even these indications were of somewhat doubtful 

 interpretation. It appears, however, that the effort of these investigators 

 was directed toward the detection of the general deformations of the 

 spheroid directly assignable to the tidal forces, and it is not clear that the 

 observed results are to be interpreted as equally adverse to the existence 

 of shorter pulsations assignable to the normal vibrations of the spheroid, 

 induced by the tidal strains. The nature and likelihood of such shorter 

 pulsations will be considered later. 



So far as opinion as to the value of the lithospheric tides is entitled to 

 weight we can not do better than to quote the conclusions of Sir George 



» Natural Philosophy, Thomson and Tait, ed. 1890, p. 418. 



* It should be understood that this is merely an individual view unsupported by the 

 expressed opinion of any special student of the tides, so far as I know, and without recog- 

 nition in the hterature of the subject. It is based on the conviction that wliile the direct 

 rise and fall of the surface of the lithosphere in response to attraction similarly affecting 

 the water tends to reduce the amoimt of the water-tides, the tilting of the lithospheric 

 bed in which the oceans lie first on one side and then on the other in the coiirse of the 

 progress of the lithospheric wave must develop an inertia tide very similar to the waves 

 produced by the rocking of artificial basins. It is also my view that the various free pul- 

 sations that may arise from the forced deformations of the Hthosphere may give impulses 

 to the waters resting in basins on its surface and that water-waves may spring from these 

 quite independently of the direct attraction of the tide-producing body, though of course 

 indirectly dependent on it. 



^ Reports to the Brit. A. A. S. on Measurement of the Limar Disturbance of Gravity, 

 York meeting, 1880, pp. 93-126, and Southampton meeting, 1882, pp. 95-119; also 

 "Tides," G. H. Darwin, 1893, pp. 108-148. 



* Das Horizontalpendel, Nova Acta Leop. Carol. Akad., 1892, vol. 60, No. 1, p. 213; also 

 Brit. Assoc. Repts., 1893; also Ueber Horizontalpendel-Beobachtimgen in Wilhelmshaben, 

 Potsdam, und Puerto Orotava auf Tenerifa, Astron. Nachrichten, vol. 103, pp. 194-215. 



' Horizontalpendel-Beobachtungen, Beitrage zur Geophysic, vol. 3, Pt. I, 1896. 



