26 NATURAL HISTORY 
* 
owing merely to the greatnefs of the heat, and the thicknefs of the 
vapour in thofe wells, and not to a fulphureous or mineral halitus : 
That it is this thick vapour which extinguifhes lights ; and that the 
tepid flream is vifible in frofty weather p : That it is from this under- 
ground heat that the Water of our Rock-Springs is never frozen : 
That we meet with the like warm watery vapour in Caves as well 
as in Wells \ To fupport this Hypothecs, he next produces ' his 
diary of the Thermometer fiifpended on different days for different 
Jpaces of time at different depths, and notes how much the Spirit in 
the Thermometer did rife by the warmth of his Well, above what 
it was in the open air before he let it down. But, with great fub- 
miffion, nothing certain can be concluded from thefe experiments in 
favour of the Hypothecs here efpoufed. 
That it is warmer under-ground than it is above, efpecially in the 
colder months of the year, in which all Morton’s experiments were 
made * ; and that at a great depth there is a greater difference, other 
circumftanccs being equal, betwixt the air underneath and that 
above, than immediately or but a few feet under the furface, muff 
be agreed ; but the queftion will remain to be decided, What this 
difference is owing to ? The Atmofphere is a congeries of many 
bodies, air, fulphur, nitre, fait, earth, and fire, and capable of 
being chilled or heated by the different difperfion or combination of 
its own ufual contents, as well as by the different influences of the 
celeftial bodies. The air therefore is fubjeft to great alterations of 
heat and cold ; but where the air of the Atmofphere cannot reach, 
a more even temperature prevails, and the cold of the common air 
fhall not affect the air of places with which it has not a free commu- 
nication any more than the heat : this is evident in palling from the 
open air into a clofe room ; and in a mine or well, where the air 
is lefs agitated by the wind, and not fo mixed with the exhalations 
which occafioned cold and froft as the air above, it is impoflible 
but it mufl be warmer for the generality, than it is upon the furface 
of the earth ; and this is all that Morton’s experiments prove. The 
ingenious and accurate Dr. Hales having placed fix Thermometers, 
one above ground, and the reft “ with their balls immerfed in the 
earth, from two to 24 inches, at different depths, when the froft 
of the winter 1724 was fo intenfe as to freeze the furface of the 
ftagnant Water near an inch thick, found that the Spirit in the 
Thermometer, which was expofed to the open air, was fallen four 
degrees below the freezing point; the Spirit of that whofe ball 
was two inches under-ground, was four degrees above the freez - 
ing point; the third, fourth, and fifth Thermometers, were pro* 
p Northamptonfhire, pag. 298. r lb. page 300. 
* Ibid. pag. 298. * From December 9, to March 10. 
portionably 
