124 
SHOUT NOTES ON PROFESSIONAL SUBJECTS. 
FH carries at its top a piece of brass F, fixed at right angles to the plane of the 
T square. This piece of brass is pierced with a cross-sight hole, and forms a 
pointer. 
On EG slides the rectangular frame ABCB , to which is attached an arm BK 
(pivoted at B ), capable of sweeping ovbr all the back of the frame. 
The top of the frame BA (from B where BK is pivoted, to A , the prolongation 
of the interior edge of BA), is graduated regularly from 0 to 600 feet.* 
The interior edge of AD is graduated as for a tangent scale for an instrument 
600 feet above sea level. 
To use the Beale. —Suppose we are in a battery 373 feet above sea level. 
Slide the frame over EG until the pointer F points to 373 on the scale BA. 
Place FH in the holder of the hydroscope, make the observation, and suppose 
FL the length of tangent scale indicated on FH. Move the arm BK until its 
centre line coincides with L on FH. Then read off the range on AB, as indicated 
by the centre line of the arm. 
The arm in the sketch points to a range of 1000 yards. Similarly, the range can 
be found from a battery at any elevation, by adjusting the frame previous to taking 
the observation. 
Proof Let F be set to any number n (less than 600). Then by the construc¬ 
tion of the scale we have this relation (see sketch of instrument):— 
FL : AM :: n : 600. 
Let BAM , CFL (see Pig. 2), represent the hydroscope in two different positions— 
A 
on a cliff BB 600 feet high, and on a cliff CB n feet high— AM , FL being the 
lengths of tangent scale in each case for the same distant ship. 
Let E be the distant ship on the water EB. Join AE, FE. Then we have, by 
similar right-angled triangles, the same relation as before given by the scale— 
FL : AM :: CB : BB :: n : 600. 
Conversely, when this is the case, FL and AM both refer to the same range. 
Therefore the range read off on AM (in the scale) is the true one for FL. 
A scale similar to the above has been made at Dover, and when used with a 
hydroscope gave good results. 
* 600 feet are taken, as it is assumed that no battery is more than 6C0 feet above sea leveh More 
or less might be taken. 
