DE. JOULE ON SOME THEEMO-DTNAMIC PEOPEETIES OE SOLIDS. 
109 
rubber tube. Mercury or water may be poured into the interstice between the glass 
tube and the wooden rod, and so serve as the means of convection of heat from the 
water with which the gutta-percha tube is filled. A stopcock 
enables one to change this water when desired. The ends of 
the rod are held by suitable clamps, which are attached, one to 
a fii-m upper support, the other to a lever of which one extremity 
has a knife-edge resting on a steel plate, and the other is fur- 
nished with a micrometer consisting of a glass plate, divided into 
2 ^ths of an inch, examined by a microscope. The weight of the 
lever alone produced a tension of 35 lbs., which could be further 
increased by the addition of weights. When a very weak tension 
was desired, a lighter lever was employed. 
61. Preliminary trials informed me of the time (never exceeding 
five minutes) required to bring the rod sensibly to the temperature 
of the water in the gutta-percha tube ; I also found that the most 
ready means of obtaining uniform temperature in this water was to 
fill the tube twice, allowing a little intervening time. The obser- 
vations of the micrometer were generally made at intervals of five 
minutes; the first observation being at a low temperature, the 
second at a high temperature, and the third at a low temperature again. The mean 
of the first and third observations gave me the length at the low temperature, the differ- 
ence between which and the length at the high temperature, gave me the expansion of 
the rod. When a high tension was employed, gradual elongation frequently took place 
for so long a time, that I did not wait till it had entirely ceased. The system of 
observation employed was, however, such as prevented error arising from this circum- 
stance ; and since the gradual elongation was more rapid as the temperature was raised, 
the precaution was taken to expose the rod to the high temperature for the same 
space of time immediately before and after each micrometer observation at the high 
temperature. 
62. Bay Wood . — The piece selected was well seasoned and very straight in the grain. 
Its length was 46 ’63 inches; the part passing through the glass tube and exposed to 
change of temperature measuring 33 inches. The diameter of the rod was fths of an 
inch, and its weight, in the first instance, about 196 grains to the foot. Mercury poured 
into the inner glass tube formed the means of conducting heat from the surrounding 
water to the rod. The following Table contains the observed expansions after various 
intervals of time : — 
Fig. 7. 
f 
P 
MDCCCLIX. 
Q 
