1819.] P>'°f' Leslie on Heat and Climate. 23 



It is an undoubted principle, that there is a continual commu- 

 nication between the hot and cold portions of a fluid ; and, 

 therefore, without appealing to actual observation, we may 

 strictly infer a regular and perpetual circulation of air between 

 the Equator and the Poles. No branch of physics is, indeed, so 

 intricate or so imperfect as the doctrine of winds. We cannot 

 judge of their direction with certainty from that which obtains 

 near the surface ; since it appears from the relations of aeronauts 

 that, in the higher regions of the atmosphere, various and even 

 opposite currents exist at the same time ; the only discovery, 

 perhaps, if it deserve that name, which has been made by bal- 

 loons. These irregular streams of air result from local and acci- 

 dental circumstances ; nor is there any reason to doubt but the 

 general sum of motion is directed between the Equator and the 

 Poles. The difference between the average temperature of dif- 

 ferent latitudes is such as to produce the exact celerity of com- 

 munication necessary for maintaining perpetually the balance. 

 The quantity of heat received from the sun, in the space of a 

 year, is annually distributed over the face of the globe. Each 

 place too must receive the same gradual increase of tempera- 

 ture, otherwise the rate of aerial circulation would in time be 

 affected, and consequently the heat differently distributed, which 

 would again restore the equilibrium. It appears, likewise, that 

 the average heat of places, on the level of the ocean, must 

 depend on their latitude. The formula which the celebrated 

 astronomer, Professor Mayer, has given for computing that 

 average, answers remarkably well. It is 84° — 52° <p 2 , <p de- 

 noting the sine of the latitude. If the theory of aerial motions 

 were perfected, a more accurate expression could probably be 

 discovered from the principles we have been establishing. — 

 America forms a curious exception to the general rule. Some 

 philosophers have ascribed the unusual cold which prevails on 

 that continent to the immense natural forests, which exclude 

 the sun's rays, and expose a great furface to evaporation.* 

 The first of these causes should be set aside ; nor can any stress 

 be laid upon the second, till it be proved that the quantity of 

 humidity evaporated in the New World exceeds what is again 

 precipitated in clouds and vapours. I am, disposed to assign a 

 different reason. Owing to some causes, which 1 will not pre- 

 tend to determine, the prevailing wind in the United States of 

 America is the north-west, which blows from the high bleak 

 territory that extends from the Lakes of Canada to the in- 

 hospitable shores of the Pacific Ocean. It is prevented, by the 

 tall ancient forests, from sweeping the surface ; and it, therefore, 

 arrives on the settlements near the coast full charged with cold. 

 Similar effects are probably produced in South Amenca.i- 



* See particularly an ingenious note, by my learned friend, Professor Robison, 

 in Dr. Robertson's History of America. 



t This explication, though conformable to the ordinary principles, is really 

 not admissible. See the Article Climate, already referred to.— A. 



7 



