121 
on the strength of materials. 
through a projection on the bed plate, large enough to admit 
a stout bolt easily through it, which again was prevented from 
turning in the hole by means of a tongue ( t ) fitting into a 
corresponding groove in the hole. So that, in order to pre- 
serve the level, we had only to move the nut to elevate, or 
depress the bolt, according to the size of the specimen. But 
as an inequality of pressure would still arise from the nature 
of the apparatus, the body to be examined was placed between 
two pieces of steel, the pressure being communicated through 
the medium of two pieces of thick leather above, and below 
the steel pieces, by which means a more equal contact of 
surfaces was attained. The scale was hung on a loop of 
iron, touching the lever in an edge only. I at first used a 
rope for the balance weight, which indicated a friction of 
four pounds, but a chain diminished the friction one half. 
Every moveable centre was well oiled. Of the resistances 
opposed to the simple strains which may disturb the quiescent 
state of a body, the principal are the repulsive force, whereby 
it resists compression, and the force of cohesion, whereby it 
resists extension. On the former, with the exception of the 
experiments of Gauthey and Rondelet, on stones, and a few 
others, on soft substances, there is scarcely any thing on 
record. In the memoir of M. Lagrange, on the force of 
springs, published in the year 1760, the moment of elasticity 
is represented by a constant quantity, without indicating the 
relation of this value to the size of the spring : but, in the 
memoir of the year 1770, on the forms of columns, where 
he considers a body whose dimensions and thickness are 
variable, he makes the moment of elasticity proportional to 
mdcccxviii. R 
