450 LECTURE XLVII. 



many ages, and with it the level of the Mediterranean also ; since the floor 

 of the cathedral at Ravenna is now several feet lower with respect to the sea 

 than it is supposed to have been formerly, and some steps have been found 

 in the rock of Malta, apparently intended for ascending it, which are at 

 present under water. 



The atmosphere is also liable to elevations and depressions analogous to 

 those of the sea, and perhaps these changes may have some little effect on 

 the winds and on the weather ; but their influence must be very incon- 

 siderable, since the addition of two or three feet to the height of the atmo- 

 sphere 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 -^ of 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. 

 These currents, as well as the general current of the sea, have been attri- 

 buted, by some astronomers, to the immediate attraction of the sun and 

 moon, and of the satellites of Jupiter, which they have supposed to act in 

 the same manner as the attraction of the sun operates in retarding the 

 lunar motions ; but the fact is, that, according to Mr. Laplace, the disturb- 

 ing force of the sun produces this effect on the moon only in proportion as it 

 increases her distance from the earth ; consequently no such retardation 

 can possibly be produced by the force of gravitation in the rotation of the 

 sea or of the atmosphere, and the whole effect must be attributed to the 

 operation of meteorological causes, producing first the trade winds, and 

 secondly occasioning, by means of the friction of these winds, a similar 

 motion in the sea. In the case of the atmosphere of Jupiter, the effects of 

 heat can indeed scarcely be supposed to be very perceptible, and the rota- 

 tion of this planet being extremely rapid, it is not at all impossible that the 

 satellites may exert an action on the atmosphere somewhat analogous to 

 the retardation of the moon's motion by the disturbing force of the sun. 



LECT. XLVII. ADDITIONAL AUTHORITIES. 



Borro, DelFlusso e Reflusso del Mare, Fiorenza, 1577. Moray on Observing the 

 Tides, Ph. Tr. 1665-6, i. 298. Colepresse's Obs. at Plymouth, ibid. 1668, iii. 632. 

 Davenport on the Tides at Tonqueen, ibid. 1684, p. 677. Halley on do. ibid. 1684, 

 p. 685. Newtoni Principia, and Halley's Remarks, ibid. 1697, p. 445. Prize 

 Essays on the Tides, by Cavalleri, Bernoulli, Maclaurin, and Euler, Hist, et Mem. 

 Prix iv. VI.. . IX. Le Seur's edition of Newton's Principia. Jones and Saumare* 

 on the Tides in the Thames, Ph. Tr. 1726, p. 68. Wright on an irregular Tide in the 

 Forth (the Leaky), ibid. 1750, p. 412. Toaldo, Tabula Barometri ^Estusque Maris, 



