DR. JOTJLE OX SOME THERMO-DTXAMIC PROPERTIES OF SOLIDS. 
Ill 
XensioD. 
Micrometer observations, 
the temperature of the 
wood being 15'’‘99. 
Micrometer observations, 
the temperature of the 
wood being 4r-7S. 
35 
0 
0 
135 
4-199 
4-381 
235 
8-539 
8-914 
335 
12-890 
13-415 
435 
17*135 
17-743 
335 
12-830 
13-392 
235 
8-579 
8-968 
135 
4-373 
4-602 
35 
0-231 
0-268 
67. IT'lSo— -^ = 17'02, and 17-743 — ^ = 17-609, which are therefore the elastic 
strains in -j^ths of an inch, produced by a tension of 400 lbs. at the respective tem- 
peratures 15°-99 and 41°-78, the length of the rod being 33 inches. These results, 
reduced to unity of length and temperature, give the difference between -000099953 
and -000103410, or -000003459, as the quantity by which the expansion by heat of 
the wood ought to be increased by adding a tension of 400 lbs. Eeferring above, it 
will be seen that this increment is actually only about -000001. I believe that two 
caiLses are concerned in producing this discrepancy. In the first place, I have reason 
to think that when tension is applied to wood and various other substances, a strain 
takes place of the nature of set besides the true elastic strain, which set is almost in- 
stantaneously taken out again on the removal of tlie tension ; also that this temporal-) 
s<«t increases with the temperature. Secondly, I think that when the expansion by heat 
of a body under tension is observed, the tendency of heat to take out set operating in a 
contrar)- direction to the expansive action, causes a slight apparent diminution of the 
latter. It will thus happen that the apparent decrease of elasticity by heat is greater 
than the real, while the apparent increase of expansion by heat, in consequence of the 
application of tension, is less than the real. These observations will be found to receive 
further confirmation from the phenomena of the elasticity of moistened wood, and also 
in the case of whalebone, a substance in which the characteristics of wood as regards 
s*'t are very strongly developed. 
68. VoL\vo\s modulus of elasticity, deduced from the above data in terms of the 
length of a rod weighing one lb. to the foot, which would be extended by unity by the 
tensile force of one lb., is 5539710 and 5354349 for the temperatures 15°-99 and 41°-78 
respectively. 
69. The same rorl of bay wood was now emjiloycd to determine the effects of hygro- 
metric condition f)n its expansion and elasticity. Mercury or water was employed as 
the medium for conducting heat between the gla.ss tube and the wood. When mercury 
was employed, some inconvenience resulted from its entry into the pores of the wood, 
ritimately I found that the heat was conveyed with sufficient rapidity when air only 
r>ccupied the narrow space intervening between the glass and the wood. 
q2 
