420 On the Resistance of Solids. 
* Experiments on the direct cohesion of bodies require to be 
made with much care, in order to obtain correct results, parti- 
cularly when they are brittle; and it is also very probable that 
the strength of bodies, to resist being torn asunder, is not ex- 
actly as their areas. For if the cohesion be as the area, it does 
not follow that the strain excited by a force tending to pull the 
body asunder should be in the same proportion. Indeed, Count 
Rumford’s experiments Jed him to conclude that the strength is 
not in the simple proportion of the area*, and Perronet found 
that when a bar of iron was strained in the direction of its 
length, a very slight incision made with a file on one side, some- 
times reduced its strength more than one-half; which could not 
have been the case had the strain been equally diffused over the 
section +. i 
Hence I conclude, that the length being taken so that the de- 
flexion t does not sensibly affect the result, the cohesion caleu- 
lated from the tratisverse strength, is as near or nearer the real 
cohesion than that obtained bv pulling the body asunder. 
Morveau very justly observes that the maximum cohesion of 
the metals ought not to enter into the calculations of the artist 
who employs them, as it is known that accidental imperfections 
always accelerate the rupture in a greater or less degree, and 
render it necessary to augment their dimensions; but it is not 
less true that their proportional tenacity being known, it would 
assist the artist in his choice, and furnish the valuable means of 
ascertaining the degree of purity, and -the qualities which are 
imparted to them in the course of their manufacture §. 
It will be seen from the following Tables, that the cohesive 
force of metals is much increased by wire-drawing, rolling, and 
hammering ; and that the strength of woods of the same kind 
is extremely variable, depending on the nature of the soil, the’ 
situation, atid the climate where they are grown. 
A complete table ought to exhibit the limits of these variations, 
and might be made to contain much useful information for the 
planter and landed proprietor,—but not till experimentalists 
give better descriptions of the specimens they try, than they have 
hitherto done. , 
In the following tables the specific cohesion of plate glass is 
denoted by unity; and, 
The cohesive force of a square inch is 9420 pounds avoirdup. 
oe oe -. ofasqu. millimetre is 6-62 kilogram. nearly. 
o ee »« of 25 squ. centim. is 16543°625 kilograms. 
ee ys «. of asquare pouce is 9906°26 Paris pounds. 
Hence the specific cohesion of a substance multiplied by one 
of these numbers will give the cohesion in the corresponding 
weights and measures. 
* Phil. Mag. x. 51. ’  CBuvres de Gauthey, ii. 150, © 
‘J On the effect of the defiexion, see Phil, Mag. xlvi. 191. 
§ Ann. de Chimie, \xxi. 190. ~ 
