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NAVAL APPARATUS, NEW. 



school management, it also disapproves of the ex- 

 clusion from a fair share in the management of 

 those parents or guardians, of whatever denomi- 

 nation, who send their children to the schools. 

 The headquarters of the League are at 140 Nas- 

 sau Street, New York city. 



NAVAL APPARATUS, NEW. Fiske's 

 Range Finder. A ready means for determin- 

 ing the exact position and range of an object 

 to be fired at, whether from a fort or war vessel, 

 is of the greatest practical importance in gun- 

 nery ; and anything that can give this informa- 

 tion at a glance, and at the same time be simple 

 in construction and of easy manipulation, ap- 

 pears at once to the practical minds of the sailor 

 and soldier whose duties are the pointing and 

 firing of the guns. Lieut. Bradley A. Fiske, of 

 the navy, has devoted considerable attention to 

 this subject, and has produced an extremely in- 

 genious instrument for use on board ship, called 

 a range finder, and another somewhat similar in- 

 strument for army use, which is called a position 

 indicator. A decidedly novel application of the 

 Wheatstone bridge as a means of measuring the 

 angles, and by means of which ranges or dis- 

 tances can be read directly from a scale, forms 

 the principle on which the whole is based. 

 Broadly considered, the method consists in de- 

 termining a fractional portion of a conductor, 

 bearing in length a ratio to the angle included 

 between two lines of sight directed upon a dis- 

 tant object, and simultaneously causing a dis- 

 turbance in an electrical balance, including the 

 conductor in its circuit, proportional to the re- 

 sistance of the fractional portion, and observing 

 the difference in potential due to the disturbance. 

 The accompanying diagram illustrates the sim- 

 ple and ingenious manner in which this is car- 

 ried out. A B is the base line, T the position of 

 a distant object, the range of which, A T, is to be 

 determined. By trigonometry, in the triangle A 



T B, A T = sit f A ^ B x sin A B T. Let C and 



D represent two telescopes pivoted at the points 

 A and B, and sweeping over arcs E and P of 

 conducting material, the arcs having their ex- 

 tremities upon the base line A B. Let the tele- 

 scope C be directed upon the point T, assuming 

 the position C' in dotted lines. Then, obviously, 

 the angle C' A C is equal to the angle A T B, 

 and the portion of the arc E included between 

 the positions C and C' of the telescope will meas- 

 ure the angle A T B. In the foregoing formula 

 the base line A B is known by measurement, and 

 the angle A B T may be observed ; and if the 

 angle A B T is, as shown in Fig. 1, a right 

 angle, then the sin A B T becomes unity. It 

 remains, therefore, to find the angle A T B in 

 order to determine the distance A T, so that it 

 becomes necessary to provide a simple and rapid 

 means of at once determining what the angle A 

 T B is. To this end, the conducting arcs E, F, 

 are connected in the manner of a Wheatstone 

 bridge, the four members of which are shown 

 respectively at a, 6, c, d. In this bridge is con- 

 nected a galvanometer in the usual way, and also 

 the battery h, the terminals of the battery wire 

 being connected to the telescopes at their pivot 

 points A, B, so that the circuit proceeds through 

 the telescopes to the arcs, and then at the arc F 

 divides through the wires b, d, and at the arc E 



divides through the wires a, c. When the two 

 telescopes C and D stand at right angles to the 

 base line, and hence parallel to each other, the 

 bridge will balance and no deflection will be 

 shown at the galvanometer. But if the telescope 

 be moved out of parallelism with the other, then 

 the bridge will be thrown out of balance and the 

 galvanometer will be deflected. The extent of 

 this deflection depends upon the length of arc 

 included between the two positions of the tele- 

 scope, and will be greater as that arc increases, 

 so that with a battery of constant 

 electro- motive force it becomes 

 possible to determine the ex- 

 tent of movement of the teles- 

 cope by simply observing the 

 indicator of the galvanome- 

 ter, which is graduated to , 

 yards. The resistance of 

 the galvanometer has 

 been neglected, and it / 

 has been assumed / 

 that the E, M, F, / 

 and internal re- / 

 sistance of the bat- / 

 tery and the re- / 

 sistance of the 

 various con- 

 tacts remain 

 constant. 



T 



A 

 / I 



FIG. 1. RANGE FINDER DIAGRAM. 



While this is not theoretically true, Lieut. Fiske 

 finds that by using storage batteries and by mak- 

 ing the contacts carefully, no appreciable error is 

 introduced. Careful experiments with this range 

 finder at sea show that the errors of the instru- 

 ment are insignificant and the indications abso- 

 lutely instantaneous. The telescope is mounted 

 upon the top of a hollow pedestal, through which 

 the wires are led, and as aluminum bronze and 

 iron are the materials used, it can be left on deck 

 without any protection other than that afforded 

 by a cover placed over the telescope. These in- 

 struments are placed one at the bow of the ves- 

 sel and another at the stern, which gives a long 

 base line ; another pair are placed one at either 

 end of the bridge, which offers a shorter base 

 line, but one of sufficient length for the accurate 



