588 LECTURE XLVir. 



able, since the addition of two or three feet to the height of the atmosphere 

 at any part can scarcely be expected to be perceptible. The height of an 

 aerial tide must be very nearly the same with the observed height of the 

 principal tides of the sea; and the variation of atmospherical pressure, which 

 is measured by the difference between the actual form and the spheroid of 

 equilibrium, must be equivalent to the weight of a column of about 10 feet 

 of air, or only -ri-^ •jf an inch of mercury. A periodical variation five times 

 as great as this has indeed been observed near the equator, where the state of 

 the atmosphere is the least liable to accidental disturbances ; but this change 

 cannot in any degree be referred to the effect of the moon's action, since it 

 happens always about the same hour of the day or night. The atmosphere is 

 also affected by a general current from east to west, like that of the sea, and 

 there is reason, from astronomical observations, to suppose that a similar 

 circumstance happens in the atmosphere of Jupiter, on account of the actions 

 of his satellites, which must be considerably more powerful thau that of the 

 mooa. 



