674 Col. roy’s Experiments jor 
I'have already had occafion to mention that a detached 
tin cafe was fometimes applied above the veffel, in which 
ftate it is reprefbnted in the view. This method was 
thought of during the operations with the long tube, in 
order to try whether the vacuum was completely main- 
tained by the temperature of the open air? For this pur- 
pofe the cafe was placed on the ftones of the yard, with 
a fmall tube inferted in it, to preferve an open paffage.in 
the middle: it was then rammed full of a compofition. 
of fait and ice; and afterwards fixed on the top of the 
long tube. The degree of cold thus applied round the 
greateft part of the vacuum, muft have been very great, 
probably near the zero of Fahrenheit; yet it produced 
no vifible alteration in the height of the column of 
O 
quickiilver, which ftill remained in boiling water below, 
and lhould have rifen, if the vacuum had been formerly 
incompleat. As it would have occafioned much trouble 
to have lengthened the feveral parts of the apparatus fo 
as to have kept the long tube wholly in boiling water, 
the counterpart of this laft experiment was not made in 
the accurate manner it ought: neverthelefs, the tin cafe, 
being emptied of its cold compofition, was placed on 
the tube as before, and filled with boiling water; which, 
joining with the intermediate fteam arifing from that in 
the veffel below, muft have kept the whole nearly in the 
fame 
