on the Strength of Materials. 163 
ground. The lever was accurately divided in its lower edge, 
which was made straight in a line with the fulerum. A point, 
or division (D), was selected, at five inches from the fulcrum, 
at which place was let in a piece of hardened steel. The lever 
was balanced by the balance weight (E), and in this state it was 
ready for operation. But in order to keep it as level as possible, 
a hole was drilied 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 (¢)- 
fitting into a corresponding groove in the hole. So that, in or-» 
der to preserve 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 re- 
lation 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 the fourth power of the ra- 
’ dius, in observing the relatious of theory and practice to accord 
with each other. This was admitted by Euler in his memoir of 
* This machine must have had a considerable degree of friction, and 
Mr. Rennie has not, apparently, attempted to determine the quantity: it 
must however have been very considerable in the high pressures. ‘The 
lever turned upon a pin similar to that used by Gauthey. (Rozier’s Journal 
de Physique, tom. iv. p. 403), which Perronet found to have much friction 
and to cause much irregularity. To remedy the défects of this machine 
another was contrived by Kondelet, in which he attempted, and it appears 
successfully, to obviate the most material defects of the old machine. The 
action was more equal on the compressed surface, and a more accurate 
measure of the strength was obtained. Rondelet’s machine is described in 
his Traité Licorique et Practique de U’ Art de Batir, tome iii. p.79.—T. T. 
1780, 
