SHORT NOTES ON PROFESSIONAL SUBJECTS. 
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liydroscope,* but the principle involved admits of similar scales being adapted to 
all instruments for measuring angles of depression. 
Description of Tangent Scale.—FR is a tangent bar with a cross piece EG 
rigidly fixed at right angles, so that the two together form a sort of T square. 
(See sketch). 
* Bell’s hydroscope consists of a long pipe BC, connecting two small cisterns AB, CD. (See 
sketch). The pipe is supported in a bed E, and when required for use is placed nearly horizontally. 
The cisterns are partly filled with water, and two floats (exactly similar), are placed, one in each 
cistern. The tops of these floats are then in the same horizontal line. 
The side of the cistern AB carries a brass holder, in which a tangent scale FR (with a sight at 
top) slides up and down. (In the sketch the brass holder is on the other side of AB, and the 
tangent scale is the one about to be described). 
The observer stands by the cistern AB, and moves the pipe and tangent scale until the sight, the 
top of the float in DC, and the water line of the ship observed, are in the same line. 
The length of tangent scale is then indicated on FR by the top of the float in AB. 
