DEDUCTIONS FROM THE TIDES. 25 



as seems to be required by the concurrent evidences of astronomical, 

 geophysical, and seismic phenomena, it seems quite inconsistent to suppose 

 that a brief deformation of the tidal sort can be other than a minute, highly 

 distributive strain, which involves no flowage motion of the molecules 

 upon one another, with the exceptions noted, and hence no friction of the 

 fiuidal type. There is a large body of geological evidence which seems to 

 indicate that the lithosphere is able to accumulate stresses for long periods, 

 which are then relieved by permanent deformations. It is difficult to 

 understand how an earth could be possessed of this ability, if it yielded 

 fluidally to such transient and moderate stresses as those of the tides of 

 the outer part of the lithosphere. We therefore assume with confidence 

 that, whatever the amount of the lithospheric tide, it is only an elastic 

 strain which relieves itself almost instantly on the removal of the force 

 which caused it and involves little friction. 



It does not appear probable, therefore, that the body tides of the earth, 

 under this view of the earth's constitution, are an efficient agency in reduc- 

 ing its rotation. 



This conclusion, however, even if fully accepted, does not appear to 

 cover the entire possibilities of the case; for, even if the primary tidal 

 deformation of the lithosphere has little or no rotational effect, it may 

 possibly give rise to pulsations in the spheroid itself which will be com- 

 municated to the water upon its surface and give rise to water-tides. If 

 the periods of these pulsations are commensurate with those of the water- 

 bodies arising from the direct attraction of the moon and sun, they may add 

 something to these by sympathetic action, even though their independent 

 value might be inconsiderable. This leads to an inquiry as to the natural 

 oscillations of the spheriod and their relations to the oscillations of the 

 lunar and solar tides. 



THE PULSATIONS OF THE LITHOSPHERE. 



It appears to be possible to reach an approximate determination of 

 the fundamental susceptibilities of the lithosphere to oscillations of differ- 

 ent classes by combining the good offices of theoretical computations and 

 observational inductions. The types of oscillation which need to be consid- 

 ered here embrace those which traverse the interior as well as the surficial 

 parts of the earth as distinct waves of propagation, and those oscillations 

 of shape which affect the form of the earth as a whole. The latter are 

 treated as harmonic pulsations and may spring either from the transmitted 

 oscillations or from differential stresses arising from variations of attraction. 

 The data relative to transmitted oscillations have been furnished chiefly by 

 seismologists; the treatment of harmonic pulsations and fundamental sus- 

 ceptibility to such oscillation has thus far been chiefly mathematical. 



Lamb, following earher work by Kelvin, has shown that several different 

 species of harmonic oscillations may arise from both the longitudinal and 

 transverse waves transmitted through the earth.^ For a steel body of the 

 size of the earth, he found the period of the slowest fundamental mode of 



* On the vibrations of an elastic sphere, by Horace Lamb. <Proc. Lond. Math. Soc, 

 vol. 13, 1882, pp. 189-212. 



