DEDUCTIONS FROM THE TIDES. 21 



DEDUCTIONS FROM THE TIDES THEMSELVES. 



As astronomical observations thus leave it uncertain at what precise 

 rate rotation is changing at the present time, it is necessary to fall back 

 upon such other evidences as the tides themselves present, and after that 

 upon the geological evidences. Each of the three fundamental divisions 

 of the earth, the atmosphere, the hydrosphere, and the lithosphere, is 

 affected differentially by the attraction of the moon and sun, and hence 

 they are all, theoretically at least, affected by the tides. They furnish a 

 suggestive combination for study in that the first is a highly fluent elastic 

 body, susceptible of great and easy changes of form and volume; the second 

 is extremely mobile, but sensibly incompressible; while the third is solid, 

 at least externally, and probably rigid as a whole and possessed of effective 

 elasticity of form. Because of the markedly different properties of these 

 three components of the earth, it would seem that comparisons of their 

 individual responses to the differential attractions of the moon and sun 

 might throw special light on tidal phenomena. 



THE TIDAL PHENOMENA OF THE ATMOSPHERE. 



Because the atmosphere is a highly symmetrical envelope, because its 

 continuity is broken by no barriers, because it is extremely mobile, because 

 it has great elasticity of volume, and because it presents greater differences 

 of distance from the tide-producing bodies than the hydrosphere or the 

 lithosphere, it would seem that it should give a tide of declared charac- 

 teristics. We are, however, almost wholly without evidences of such a tide, 

 notwithstanding the large mass of barometrical data at command. These 

 data stretch over a long term of years and are refined enough to show 

 several small periodic oscillations, but none of these, at least none of those 

 commonly recognized, are timed with the moon. Atmospheric tides play 

 no part in the science of modern meteorology. Laplace discussed the 

 tides of the atmosphere briefly and theoretically and found that if the sun 

 and moon were in the plane of the earth's equator and if the two bodies 

 were in the same line and at their mean distances, the variation of the 

 barometer would be 0.63 mm.* Darwin, without entering upon their 

 discussion, expresses the opinion that they are undoubtedly very minute.^ 

 Other methods of estimating the atmospheric tides support Laplace in 

 showing that the amount of the forced tides should be just within the 

 limits of observation, from which it is inferred that they should become 

 quite appreciable if they were much reinforced by the cooperation of free 

 waves. The chief light which their scantiness seems capable of throwing 

 on the general problem in hand is that which bears on the dependence of 

 the actual tides upon the reinforcement of the forced waves by the com- 

 mensurable action of the free waves that spring from them. 



The best observational data relative to the rate of propagation of a 

 free atmospheric wave arising from a forced oscillation are those furnished 



» M^canique Celeste, Pt. I, Bk. IV, and Bk. XIII, vol. 5, p. 337. 

 'Enc. Brit., "Tides," p. 353. 



