i6 INTRODUCTION. 



portion of cold air to come in conta(!t with the 

 heated furface. 



But ftill it is immaterial, whether the heat of the 

 fun be excited more in the earth, than in the air ; 

 for whichever has the largefl: portion, will in the end 

 communicate a part to the other, and fo reflore the 

 balance. The fame obfervation applies to fuch caufe* 

 of cold as may operate at the furface of the earth j 

 as evaporation, and that taken notice of by Mr. 

 Wilfon. The air, therefore, near the furface of the 

 earth, will fhow by a thermometer in the fhade, 

 nearly, if not exadly, the fame degrees of heat that 

 the fun communicates to our terreftrial globe : And 

 if a mean of the heats thus fhown, be taken for the 

 year round, and we penetrate into the earth to that 

 depth that is no longer affeded either by the daily^ 

 monthly, or annual variation of the heat j the tem- 

 perature at fuch a depth fhould be equal to the an- 

 :iiual mean above mentioned. 



To afcertain this with the utmoft precifion, it mud 

 be obvious that numerous obfervations ihould be 

 made every day, correfponding to the frequent 

 changes of temperature, which are known to happen 

 in the twenty- four hours in all climates ; and upon 

 thefe a daily mean fhould be taken, and the annual 

 mean deduced therefrom. This has not yet been done : 

 But when we have obfervations, from which a mean 

 temperature can be deduced with any degree of 

 certainty, it will be found not to differ greatly 

 from the heat of deep caves or wells in the fame 

 climate. 



If 



