550 LECTURE LVI. 



In consequence perhaps of friction in its passage, it gradually loses its im- 

 petus towards the west, and at the equator is nearly a south wind ; but in 

 proceeding north from the equator, it becomes, from an excess of rotatory 

 motion, a south west wind, which blows into the Arabian gulf and the bay 

 of Bengal. Both these winds are however variously modified by the par- 

 ticular situations of the islands and continents. From October to March, 

 on the contrary, the sun having south declination, the south east trade wind 

 stops at 10 south latitude ; the trade winds on the north side of the equator 

 are as usual north east ; and beyond the equator they become for some 

 degrees north west, the circumstances being the reverse of those which 

 happen in the summer months, at greater distances, on the other side of 

 the equator. (Plate XLII. XLIII.) 



The last fact is the simplest of all. The land and sea breezes are pro- 

 duced by the ascent of the air over the land in the day time, while the land 

 is hotter than the sea ; and its descent at night when the land is become 

 colder : hence the breeze comes from the sea by day, and from the land by 

 night. 



The violent agitations of the air, which constitute hurricanes and whirl- 

 winds, occur more commonly in tropical climates than in others. The 

 causes of these storms are little understood : their course is said to be gene- 

 rally opposite to that of the trade winds ; but tornados, which are less re- 

 gular hurricanes, originate indifferently from every quarter. 



The variations of the weight of the air, w r hich occasion the winds, and 

 other changes in its density, which are the effects of the winds themselves, 

 are indicated by the height of the barometer, which is in general the more 

 variable as the winds are more liable to sudden changes. Hence in the neigh- 

 bourhood of the equator the height of the barometer is scarcely ever a quarter 

 of an inch more or less than 30 inches, which is very nearly its mean height on 

 the level of the sea in every part of the globe : in Great Britain it is some- 

 times as low as 28 inches, but never higher than 31. We have already 

 seen that the elevation of any place above the sea reduces the height of the 

 barometer according to a law which is determined by the general properties 

 of elastic fluids : thus, at an elevation of 1 mile above the sea, the mean 

 height of the barometer is 24% inches, and at 2 miles, 20 inches only. The 

 use of the barometer, in foretelling variations of weather, is perhaps more 

 limited than has sometimes been supposed ; but by a careful observation, 

 conclusions may be drawn from it, which may in many cases be of con- 

 siderable utility : and it has even been applied with success, by some late 

 navigators, to the prediction of changes of wind, at times when they could 

 not have been suspected from any other circumstances.* 



LECT. LVI. ADDITIONAL AUTHORITIES. 



Meteorology in general. Richard, Hist. Naturelle de 1'Air et des Meteores, 10 

 vols. 12mo, Paris, 1770-1. Toaldo, Saggio Meteorologico, 4to, Padova, 1770. 

 La Meteor. Applicata, 4to, "Venezia, 1786. Deluc, Idees sur la Metgprologie, 

 2 vols. 1786-7. Cotte, Traite de Met. 4to, Paris, 1774; Mem. sur IziMeteor. 

 2 vols. 4to, 1778. Horrebow, Tractatus Historico-meteorologicus, 4to, Copenhag. 



See D'Alcmbert, Reflexions sur la Cause Generate des Vents, 4to, 1747. 



