SJ 4 Mr. Thompson’s Experiments 
In order that every part of the apparatus employed in thefe 
experiments might be as perfect as pofiibie, all the more deli- 
cate parts of it were executed by Mr. fraser, mathematical 
inftrument-maker in Duke’s Court, St. Martin’s Lane, and* 
among the reft, all the contrivances juft deferibed relative to 
the vent. 
The velocities of the bullets were determined by means of a 
pendulum, according to the method invented by Mr. robins. 
The pendulum I made ufe of (fig. 6.) is compofed of a cir- 
cular plate of hammered iron (ff), 13 inches in diameter, and 
0,65 of an inch thick, to which is firmly faftened a bar of iron 
( [b , <?) 56,5 inches in length, 2,6 inches broad, and half an 
inch in thicknefs, by which it is fufpended by means of two 
pivots (c/, <?) at the end of the bar (r), and at right angles to 
its length. Thefe pivots being very accurately finifhed, and 
moving on polifhed grooves, which were kept conftantly' oiled 
to leften the friction, the vibration of the pendulum was very 
free, as appeared by the great length of time its' vibrations con- 
tinued after it had been put in motion, and was left to itfelf. 
To the circular plate of 'the pendulum, targets of circular 
pieces of wood of different thicknefles were fixed, which in the 
courfe of the experiments were often fpoiled and replaced ; and, 
in order to mark the weight and dimenfions of the pendulum 
in each experiment, the pendulums are numbered according to the 
different targets that were made ufe of; and the weight and 
dimenfions of each pendulum are fet down in a table at the end 
of the defeription of the apparatus. 
The target of the pendulum N° 1. was made of a circular 
piece of elm-plank, 3I inches thick, and equal in diameter to 
the iron plate of the pendulum to which it was fixed | but this 
target being too thin was very foon ruined. 
The 
