HEAT OR CALORIC. 69 



the equilibrium : during his passage to the southern tropic, the process is 

 reversed, and the wind blows, for six months, the other way. 



Sea breezes by day, and by night, in hot climates, and in hot 

 weather in temperate climates, depend upon the same principle. 

 The trade winds are caused by the tendency of the cold currents to 

 restore the pressure, occasioned by the rarefaction of the air, within 

 the tropics, from the perpetual presence of the sun in that region. 

 Hence, the currents which the atmosphere pushes in, from the north 

 east and the south east, are, at the equator, blended into one, which 

 follows the apparent course of the sun. The heated air which rises, 

 is in the mean time diffused over the upper regions of the atmosphere, 

 flows north and south, is chilled >and condensed, and falls in the tem- 

 perate and polar regions, to go through the same round again.* 



(q.) Currents upward and downward, both in gross and aerial 

 fluids, produce a vast and salutary effect on the comfort of the globe. 



The warm ocean imparts its heat to the chilled land, of the polar 

 regions, and the hot land of the tropical countries gives its heat to the 

 water of the cool ocean ; the monsoons and trade winds and common 

 winds produce a similar effect in the atmosphere. f 



Without currents, the atmosphere would become fatally hot, in tor- 

 rid, and fatally cold in frigid climates; and similar inequalities in the 

 ocean and other great waters would be deadly to the aquatic animals. 



(R.) RADIATION OF HEAT is ITS (apparently) INSTANTANEOUS 



PASSAGE THROUGH TRANSPARENT MEDIA. 



We can perceive no progress, and therefore regard the passage as 

 instantaneous : there can be no reasonable doubt that it passes as ra- 

 pidly as light. 



(s.) Caloric or heat radiates from the sun, from fires, and volca- 

 nos, and probably from all bodies. All our experience confirms this 

 statement, and particular experiments to prove it will be mentioned 

 hereafter. 



(t.) Solar heat radiates more or less, through all transparent 

 media, whether solid, fluid or aerial, and generally without heating 

 them materially. J 



(u.) Culinary, or artificial heat radiates only through air, and 

 other aerial fluids, and not through transparent solids, or transpa- 

 rent gross fluids, as water, alcohol, fyc. The cause of this difference 

 is not known. 



(v.) The transparent bodies through which artificial heat does not 



* See Dr. Hare's essay on the gales of the Atlantic States of N. Ain.Am. Jour. 

 Vol. V, p. 352. 



t Murray, 2d Edit. Vol. I, p. 276. 



\ The lower regions of the air would be quite as cold as the upper, did they not 

 receive heat from the earth. 



There is a difference in this respect, among media; water arrests abgut half the 

 I'ays, and alcohol more than half, and of course heat is acquired by these fluids, 



