7'io Col. rov’s Experiments for 
the mean of three experiments) as 484 to 252, when 
prefied with only one-fifth of an atmofphere. 
The 
i pended around it. Two thermometers were made ufe of; the long one, whofc 
ball flood at the bottom; and a fhort one at the top, that defcended no lower 
than juft to be immerfed in the water. By fome of the firft of thefe experi- 
ments, the lamps not being placed dire£lly at the bottom, water was perceived 
to be a very bad conductor of heat; for it would boil violently at the top, and 
the fhort thermometer there would mark 21 2°, while the long one would only 
mark 185° or 190° at the bottom. By flow degrees the heat would neverthelcfs 
defeend, and in the fpace of fifteen or twenty minutes the whole column would 
become of the fame uniform temperature. But when the apparatus was adapted 
for experiments on air denfer than the atmofphere, in which cafe a plate of tin 
was foldered over the hole at the bottom, that it might be placed on a flrong 
fire, the heat was then greatefl below, and the long thermometer w r ould mark 
upwards of 215 0 , while the fhort one flood at 209° or 210°. By defifling from 
blowing the fire, or removing a part of it, the particles of water fullering the 
greatefl heat would afeend, mix with the reft, and for fome little time make 
the whole column of an uniform temperature. But the fire being totally 
removed, the top of the column in cooling was always hotteft; wherefore, in all 
thefe experiments, whether on denfe or rare air, great care w T as taken to mix the 
water thoroughly. 
From Mr. de luc’s book it appears, that M. amontons found the effe£l of 
heat on the air confined in his thermometer, which feems to have been the fame 
fort of inflrument with the manometer, proportionable to the weight with 
which it was loaded. By this he could not mean that, being of a double den- 
fity, it had twice the expanfion. I apprehend it mull here be underftood, that the 
fpaces the air occupied, were inverfely as the weights. That being preffed with 
a double weight, it only filled half the fpace; or with half the weight, a double 
/pace. This is no doubt nearly, though not accurately, the law that it follows. 
F rom thefe experiments it appears, that there is little difference in the a£lual ex- 
panfion or elaftic force of air, preffed with an atmofphere -f- or — one- third part: 
yet, when it is rendered extremely rare, its elafticity is wonderfully diminifhed. 
*1 here feems likewife to be a vifible diminution in its expanfion, when loaded 
with two atmofpheres and a half. Some of the tubes that I ufed were near nine 
feet 
