47 S ' Temperature of the [Book V. 



rays of the fun, but conducb it through its own fub- 

 ftance very flowly to any great depth j whereas water, 

 from its tranfparency, receives heat from light but 

 flowly; but the heat is diffufed through the whole 

 mafs with gitat rapidity. Dr. Hales relates, that in 

 Auguft, 1724, when the air and the furfaoe of the 

 earth were both at 88, a thermometer, placed at only 

 two inches depth in the ground, flood at 85, another 

 at fixteen inches at 70, and another at twenty-four 

 inches at 68. The two laft preferved the fame tem- 

 perature day and night to the end of the month, and 

 then only fell to 63. On the -26^1 of Odober, a ther- 

 mometer expofed to the air by the fame philofopher, 

 (lood at 35 5, but one funk two inches in the earth 

 was heated to 43 85, another funk fixteen inches 

 reached 48 8, and one at twenty-four inches 50. 

 He even found, that between the i ft and 21! of No- 

 vember, when the external air was ac 27% a thermo- 

 meter at twenty four inches depth ftood at 43 8; but- 

 from March to September, the following year,, the ex- 

 ternal air was much warmer than the earth at fixteen 

 inches or two feet ; but the fcafon was rainy, and the 

 evaporation being confiderable, prevented rhe earth 

 near the furface. from being confiderably warm.'.'d. 



From thefc experiments it r*ppears, that the furface 

 of the earth may be confiderably heated, and yet that 

 the heat fhall not penetrate to any confiderable depth; 

 it appears alfo, that the earth parts with its heat with 

 difficulty to the air, and will retain its natural temp-*- 

 rature, which is between 40 and 50% at a very fmall 

 depth beneath the furface, even when the air is below 

 the freezing point. In water, on the contrary, the 

 heat is not accumulated in a particular part, but is 

 equally diffufed through the whole mafs, and the tem- 

 perature, if the furface is extenfive, will be more in 

 4 agreement 



