23 6 Mr. Thompson’s Experiments 
e , is the circular plate of the pendulum, to which 
j] g , two circular targets, are faftened by fcrews, and by 
means of the iron ftraps, i, 2, 3, 4, which are nailed to the 
edges of the targets. 
h y ky are the hooks which ferved inftead of grooves to receive 
the pivots, Cy dy of the pendulum. 
The hooks were firmly fixed to the horizontal beam R. S. 
which Supported the whole apparatus by means of three lcrews 
niy fly Oy which pafled through three holes in the plate that 
connefts the two hooks. When the hooks were faftened to the 
beam, the middle fcrew, riy was firft put into its place, and 
the pendulum was allowed to fettle itfelf in a pofition truly 
perpendicular, after which the grooves were immoveably fixed 
by means of the fcrews ifiy 0 . 
The chord of the arc, through which the pendulum afcended 
in each experiment, was meafured by a ribbon, according to the 
method invented and defcribed by Mr. robins. 
The recoil was meafured in the following manner. The 
barrel was fufpended in an horizontal pofition (and nearly in a 
line with the center of the target) by two fmall pendulous 
rods, 64 inches in length, and 25,6 inches afunder; which 
being parallel to each other, and moving freely upon polilhed 
pivots about the axes of their fufpenfion, and upon two pair of 
trunnions that were fixed to the barrel, formed, together with 
the barrel, a compound pendulum ; and from the lengths of 
the vibrations of this pendulum, the velocity with which the 
barrel began to recoil, or rather its greateft velocity, was deter- 
mined. 
But in order that the velocity of the recoil might not be too 
great, fo as to endanger the apparatus when large charges were 
made 
