12 
tlie velocity increased the temperature diminished, at first 
rapidly and then in a more steady manner. The first dim- 
inution might he expected from the fact that the velocity 
was not as yet equal to that at which the resistance of 
friction is sensibly equal to the square of the velocity as 
previously explained. The steady diminution which con- 
tinued when the velocity was greater was due to the cooling 
of the tube. This was proved to be the case, for at any 
stage of the operation the temperature of the emerging air 
could be slightly raised by increasing the heat under the 
water so as to make it boil faster and produce greater agita- 
tion in the water surrounding the tube. This experiment 
was repeated with several tubes of different lengths and 
characters, some of copper and some of brass, with practically 
the same results. I have not however as yet been able to 
complete the investigation, and I hope to be able before long 
to bring forward anotlier communication before the Society, 
I may state that should these conclusions be established, 
and the constant B for different fluids be determined, we 
should then be able to determine, as regards length and ex- 
tent, the best proportion for the tubes and flues of boilers. 
Dr. Joule made a further communica- 
tion respecting his mercurial air pump 
described in the Proceedings for Dec. 24, 
1872; and Feb. 4, Feb. 18, and Dec. 80, 
1878. He had successfully made use of 
the glass plug proposed in the Proceedings 
for Feb. 4, 1873. This he constructs by 
blowing out the entrance tube and grind- 
ino' the bulb thus formed into the neck 
of the thistle-shaped glass vessel. To collect 
the pumped gases he now employs an 
inverted glass vessel attached to the 
entrance tube and dipping into the mer- 
cury in the upper part of the thistle glass. 
THE SJZE 
OHE FOORTH 
