through fully two inches, and a pool of mercury two inches deep and a square foot in area weighs 145 pounds; hence, when the barometer is very high, every square foot of the earth-surface supports about 140 pounds more than if it is low; and 140 pounds to the square foot is 1,800,000 tons to the square mile.
Now, rocks are not absolutely rigid against flexure, certainly less so than most of the metals, and these enormous weights have to be supported by the rocks. Taking a probable estimate for the elasticity of rocks, I have made some calculations as to the amount of effect that we may expect from this shifting of weights, and I find that it is likely that we are at least three or four inches nearer the earth's center when the barometer is very high than when it is very low.
It may be that the incessant straining and unstraining of the earth's surface is partly the cause of earth-tremors, and we can at least understand that these strains may well play the part of the trigger for precipitating the explosion of the internal seismic forces. The calculations also show that near the sea-coast the soil must be tilted toward the sea at high-water, and that the angle of tilting may be such as could be detected by a delicate instrument like that of M. d'Abbadie.
This breathing of the solid earth seems to afford a wide field for scientific activity. It would be premature to speculate as to how far it will be possible to educe law from what is now chaotic; but it is clear that the co-operation of many observers will be required to separate the purely local from the true terrestrial changes. The directors of astronomical observatories have peculiar facilities for the study of displacements of the vertical, and it is to be regretted that hitherto most of them have been contented to banish, as far as may be, the troubles caused in their astronomical work by earth-tremors and displacements of the vertical.—Fortnightly Review.
- "Second Report to the British Association on Lunar Disturbances of Gravity." 1882.