THEORY of RAIN. 73 



reafoning from our obfervations, a fouth wind and a north 

 wind, things as different in their qualities as oppofite in their 

 directions, will fometimes be confounded. 



THE practical obfervation to be now made from this is, that 

 we fliould always allow the appearance, with regard to the di- 

 rection of the wind, or place from whence it came, to be cor- 

 rected by thofe concerning the temperature of this fluid, in re- 

 lation to heat and cold, fo far as there is reafon to conclude 

 that this indication of the thermometer is a thing of greater 

 certainty than that of the wind's direction. 



THERMOMETRICAL obfervations, with regard to the tempe- 

 rature of the atmofphere, are, from their nature, variable. The 

 furface of this earth is heated by the influence of the fun ; and 

 the atmofphere, in contact with this furface, is heated by com- 

 munication of temperature : Or, in like manner, it is cooled, 

 upon another occafion, when it happens to be warmer than the 

 furface of the earth. But, whatever may be the temperature 

 of the atmofphere, there is, in general, a certain allowance to 

 be made for the diurnal influence of the fun j and this is found 

 out, by experience, both with regard to its quantity upon the 

 fcale of the thermometer, and to the time of its periodical ap- 

 pearance in the rotation of the earth. 



HAVING thus learned to make allowance for the diurnal va- 

 riation of the thermometer, its variable temperature, as an in- 

 dication in our meteorological journal, is next to be confidered, 

 and the caufes of that change, It muff be evident, that nothing 

 can contribute fo much to change the temperature of the atmo- 

 fphere, as change in the direction of the wind, fuppofing, that, 

 in our obfervation, it gives no falfe indication. It is impof- 

 fible, that the fouthern atmofphere mould be tranfported over 

 this ifland, without giving heat above the mean temperature of 

 the feafon at which the obfervation is made ; or that the wind 

 ihould come directly from the north, without producing the 

 oppofite effect. Therefore, we are obliged to eftablifh this 



K. principle, 



