1818.] Distribution of Heat over the Glohe. 179 



both depend upon many local circumstances, such as the con- 

 stitution of the soil, and the disposition of the radiating surface 

 of the globe ; and it frequently requires much judgment and 

 discrimination to decide what circumstances are to be taken into 

 account, and what are not connected with the inquiry. The 

 object is to ascertain the quantity of heat which every part of 

 the globe receives annually, and which is of actual use to agri- 

 culture and the well-being of the inhabitants ; not what depends 

 solely upon the action of the sun, its height above the horizon, or 

 the extent of its semi-diurnal arcs. The temperature of a climate 

 depends both upon the action of the sun, and of various extrinsic 

 causes ; among these may be enumerated the mixture of the 

 temperatures of different latitudes, produced by winds, the vicinity 

 of the sea, the nature of the soil, the presence and peculiar form 

 of mountains, and the existence of large tracts of snow or masses 

 of ice. 



In distinguishing between the solar and the real climate, we 

 must remember that the local causes which modify the sun's 

 action are themselves only secondary causes, effects which the 

 motion of this luminary produces in the atmosphere. Many of 

 the local or disturbing causes are necessarily connected with the 

 nature of heat, and are felt over every part of the globe. The 

 ocean tends to equalize the temperature of all the different 

 regions by the mobility which necessarily belongs to it,' and the 

 currents of warm air which always flow from the equator to the 

 poles, tend to diminish the rigour of the countries of the north. 

 In estimating the action of the sun alone on the earth, we must 

 have recourse to theory : this will not express the actual height 

 of the thermometer in different situations, but it will show the 

 relations between the mean temperature of different regions. 

 By comparing the results of the calculation, not to the mean 

 deduced from observations made in different longitudes, but to 

 the mean temperature of a single point taken at the surface of 

 the earth, we may proceed in our examination of what depends 

 upon the sun, and upon all the other influences, solar or not 

 solar, local, or such as extend to considerable distances. In this 

 way we shall be able to form an interesting comparison between 

 theoiy and experience. 



For the first philosophical ideas upon the subject of solar heat, 

 we are indebted to Halley. Marian afterwards extended our 

 knowledge of solar action ; but he fell into some considerable 

 errors, which were rectified by Lambert, who instituted some 

 important calculations, which, however, do not always accord 

 with actual experience, and which, indeed, depend almost. 

 entirety upon mathematical principles. After him, the subject 

 was taken up by Moyer and by Kirwan ; and they proceeded 

 more upon observation, at least upon the method 01 endeavour- 

 ing to approximate to a true system by collecting observations, 

 and employing them to correct the theory- In speaking of the 



M 'J 



