302 Mr. Thompson’s Experiments 
better for the axis of the pendulous rods to reft upon level 
pieces of wood or iron, than for them to move in circular 
grooves : only care muft be taken to confine them by ftaples or 
home other contrivance, to prevent their flipping out of their 
places. 
The trunnions, by means of which the barrel is connefted 
with the pendulous rods, and upon which it is fupported, 
Ihould be as fmall as pofiible, in order to lefien the friction ; 
and for the fame reafon they ihould be well polifhed, as well as 
the grooves that receive them. They need not be caft upon the 
barrel, but may be ferewed into it after it is finifhed. 
In making the experiments,, regard muft be had to. the heat 
of the barrel, as well as to the temperature of the atmofphere ; 
for heat and cold, drynefs and moifture, have a very fenfible 
effedt upon gun-powder to increafe or diminifti its force. If 
therefore a very great degree of accuracy is at any time re- 
quired, it will be beft to begin by firing the piece two or three 
times merely to warm it ; after which three or four experi- 
ments may be made with ftandard powder, to, determine anew 
the proof mark (for the ftrength of the fame powder is dif- 
ferent upon different days) ; and when this is done, the experi- 
ments with the powder that is to be proved are to be made, 
taking care to preferve the fame interval of time between the 
firings, that the heat of the piece may be the fame in each 
trial. 
If all thefe precautions are taken, and if the bullets are of 
the fame weight and dimenfions,. powder may be proved by 
this method with much greater accuracy than has hitherto been 
done by any of the common methods made ufe of for that 
