164 
MINUTES OF PROCEEDINGS OF 
this once or twice, as it becomes more or less covered with an oxide, owing 
to the spark caused by the disruption of the current when the leaden weight 
is taken off. It sometimes happens also, unless care be taken, that some of 
the mercury splashes out of the little cup when the weight falls. 
This weight should have a hemispherical bottom; and when it gets 
flattened, by constant falling on the steel blade, it should be rubbed against 
a piece of sand paper held in the hollow of the hand; a mixture of lead and 
antimony is the best material of which to make this weight. All the 
several connexions should be kept well polished, and the insulation of the 
wire targets complete. 
I have sometimes found a target become uninsulated by being spattered 
with the grease contained in the lubricating wads, used with rifle guns. 
Care must be taken that all the pressure screws bite the wires firmly; the 
action of the disjunctor sometimes loosens the pressure screws on that 
instrument. 
The table should be truly level, the instrument carefully levelled; and the 
pendulum adjusted in the half-arc of oscillation. 
There must be no friction of the vernier against the limb, and the tension 
of the forked spring must be nicely regulated. 
The resistances in the first two circuits should be as fine as possible, 
and the battery which supplies the currents should be in perfect trim, 
well amalgamated, and the acid fresh and good. The batteries should 
be prepared at least a quarter of an hour before using the instrument. 
The operator must bear in mind that he is conducting a philosophical 
investigation which requires skill and care in order to render the results 
trustworthy. 
Use of the Table. 
29. The following is an application of the calculation, and the manner 
of employing the table ;* the vernier reading 0 o, 05. 
110-pr. Armstrong Gun. 
Instrument 32. 
A', or arc obtained by firing . = 109 o, 05 
A, or arc obtained by disjunctor . = 41-60 
To obtain the time corresponding to the arc A', the number corresponding to 
the 109th degree is sought in the column “Total Durations,” and found ... = 0"'24i0650 
To this must be added the time corresponding to -05 of the 110th degree; the 
“Partial Duration” corresponding to the 110th degree is 0 //, 001743; mul¬ 
tiplying this by '05 we obtain .^. = 0"-000086 
A' = 0 // *240736 
Proceeding in the same manner to obtain the time corresponding to the angle 
A, we find, 
For 41° ... 0"-132592 
For *60 . 0-001728 x -60 = 0"-001036 
- A = 0"-133628 
Difference = (A 1 — A) . = 0"-107108 
To calculate the velocity of the projectile from the time obtained by the 
instrument, it suffices to divide the distance between the two targets by this 
time, to give the velocity of the projectile at a point in the trajectory equi¬ 
distant between the targets. Supposing the targets to be 120 ft. apart, and 
* Vide Captain Noble’s Report, Appendix IY. p. 149. 
