THE ROYAL ARTILLERY INSTITUTION. 
195 
object-glass requires opening; the wires can always be seen when on 
the sun. 
The wires are laid on the sun so that the horizontal wire splits the sun, 
while the vertical wire touches its right edge. 
This being done at both guns, the observers follow the sun with the 
screws of the piece, calling “ on.” 
The officer halts them both at the same moment. The cross telescopes 
are then laid upon each other as usual, and the numbers recorded; this 
is, say, five times repeated; then the observers are interchanged (to get 
rid of any personal error) and five more observations taken. 
The totals of the 10 observations are added together and divided by 10, 
and the result should theoretically amount to 100 (or better, practi¬ 
cally, 99|). 
On the word “ halt, ” each observer should throw up the hand on 
the traversing screw; the officer should also ask both if the observation 
was good, and omit all bad observations. 
This test can be made in a quarter of an hour, and is then infallible ; 
one good observation takes about two minutes. The moon may also be 
used without shutters or glasses for the same purpose. 
The coloured glasses must on no account be placed over the object 
glasses, as then all the embarrassments of unequal refraction would be 
introduced. 
The guns may be at any distance apart; 40 yards for choice. 
If the shutters are lost, a piece of paper with a hole might be 
substituted. 
The Four Angle Test. 
Choose two distant points roughly, at right angles to each other, A 
and B. Lay either gun (the left) on one point A, then dress the 
second gun on B, about 45 yds. from the first gun. 2,000 yds. is a 
good distance for A ; 900 for B. 
The guns will be dressed, when both being laid on A, the centre of 
the white of the right instrument is seen just under B, looking 
through the short telescope of the left gun. 
Take a pair of angles as usual, say 43 and 52 are the gun numbers, 
now, lay the short telescope of the left gun on B ; its angle (or gun 
number) is, say, 42. Shift the left finder to the right gun, lay its tele¬ 
scopes on A and B, the new angle is, say, 4. 
Write down these numbers as below, subtract 42 from 4, adding 100 
to the 4 if smaller than the 42, but not otherwise. 
4 
42 
Besult 62 
Add 62 to the ordinary observations, 43 and 52. 
43 
52 
62 
Second result 157 
