^<S|^ I.KCTOllE XVI. 



greatest at the equator, and is directed eastwards, the air coming- 'from thfc 

 poles has of course a relative motion westwards ; and 'hence the joint motion 

 of the current is directed, in the northern hemisphere, from north east to south 

 •west, and in the southern, from south -eaist to north west. Dr. Hjrlley sup- 

 posed that the air was made in some measure tofoilowthe son roimd rhecartA, 

 simply by means of the expansion of the atmosphere, which takes place im- 

 mediately under him, and accompanies him round the globe ; but it does 

 rot seem evident that the air could have any greater tendency to follow the 

 sun than to meet him. Astronomers have, however, deduced an additional 

 cause for an easterly wind from the attractions of the moon and of tlie sun, 

 ■which -appeair, from the laws of gravitation, to liave a slight tendency to 

 retard the rotatory motion of the atmosphere: and a similar instance has 

 been observed in the motions of the atmosphere of the planet Jupiter, by 

 means of the appearances of spots of different kinds on his disc, some of 

 which seem to revolve less rapidly than the body of the planet. At so 

 great a distance, the influence of the sun's heat must be comparatively incon- 

 siderable, and the want of a tendency in the spots towards the equator appears 

 to show, that the atmosphere, iii which they float, is not put in motion by tlie 

 same causes, which we have supposed to be most concerned in the production 

 of our own trade winds. It has been remarked that the friction of the atmo- 

 sphere, thus retarded by the attraction of the sun and moon, must in the 

 course of ages have impaired the uniformity of the earth's diurnal motion ; 

 and it has been observed, on the other hand, that even this effect would be 

 partially counteracted by the gradual filling up of valleys, by means of the 

 descent of the superficial parts of mountains, which, at a greater distance 

 from the centre, were revolving with a rapidity somewhat |>reater than the 

 valleys in which they are deposited; but probably neither of these changes 

 would become sensible in millions of years. 



The second circumstance is easily explained by the greater heat of the northern 

 than of the southern hemisphere ; so that instead of coinciding with the equator, 

 the neutral portion of the atmosphere lies between 3° and 5° of north latitude ; 

 the nor.h east wind not reaching the equator, and the south east continuing 

 about 3" beyond it. But the situation of the neutral portion varies with the 

 sun's declination, accordingly as different parallels of latitude become in 

 succession somewhat hotter than the neighbouring parts. Where the 



