January 24, 1890.] 



SCIENCE. 



59 



operating the arm i' a like effect is produced in the member b. 

 The resistances or lengths of the members c and d remain un- 

 changed. 



Referring now to Fig. 3, there is shown an arrangement which 

 forms the basis of the specific embodiment of the invention, more 

 particularly hereinafter described. In said Fig. 3 the arc h is con- 

 nected at its respective ends/ J to the members a c, and the arc 

 h' is similarly connected aty J' to the members b d. The battery- 

 wires connect to the pivots //' of the arms i i' , as before. Now, 

 when the arm z' is moved from its middle position on its arc toward 

 7, less resistance is caused in the member a, and more resistance 

 in member c ; and when moved in the opposite direction, the re- 

 verse occurs. So, also, a similar effect is produced by moving arm 

 i' ; and thus the resistance offered by all four members of the 

 bridge may be affected instead of that due to only two of them, 

 and differential results may be obtained, as will more fully be ap- 

 parent in the following description of a device for measuring dis- 

 tances, such as a range-finder for guns. 



Refer) ing to Fig. 4, let T be the position of the object the dis- 

 tance of which from the point A it is desired to ascertain. Let 

 AB be any short base-line. Draw AC at right angles to BT, EA 

 parallel to BT, and prolong AT as to D. By trigonometry 



AC=ATsinATC * , j AC = AB sin ABC, whence 



AT = AC cosec ATC ( ^"° \ AT=AB sin ABC cosec ATC. 

 AB, being the measured base-line, is known, and the angle ABC 

 at the point of observation is easily determined, so that the angle 

 ATC remains to be found ; but ATC = DAE, and DAE is sub- 

 tended and measured by the arc GH. Arc GH=arcyH — arc/G, 

 and arcyH = arc J'K : hence arc GH = arc7'K — arc/G. 



In Fig. 5 the diagrams Figs. 3 and 4 are combined ; i i', as be- 

 fore, being swinging arms traversing the arcs h h', and the con- 

 nections a b c d qI the bridge being present also, as before. Let 

 the arms i and i' represent alidade-arms or telescopes, both 

 directed upon the object T. The arcsy'G andy'K not being equal, 

 the bridge will not balance ; but when the telescope i is moved to 

 the line EH, then the bridge will balance ; but the distance thus 

 moved is the arc GH, the length of which may be read off from the 

 arc h itself. It will be seen, therefore, that the operation of de- 

 termining the distance AT becomes, by the aid of this apparatus, 

 exceedingly simple. The observers at the respective telescopes i 

 and i' direct their lines of sight upon the object. The observer at 

 z' notes the angle 7 AG, or length of arcy'G. He then moves the 

 telescope / unlil the galvanometer _§-', which may be placed con- 

 veniently near his position, shows no deflection, and notes the 

 angley'AH, or length of arc/H. The difference between the arcs 

 ?G andyH equals the arc GH, whence the angle ATB, and hence 

 the distance AT, is found by the observer at the arm i, or, in other 

 words, by an observer at the base-line. The disposition of the 

 apparatus whereby an observer at a point distant from said base- 

 line may at once read off the distance AT from a suitable scale 

 will now be explained. 



Referring to Fig. 6, the members a and b of the bridge are con- 

 nected to opposite extremities of a bar m n of conducting material 

 and the members c d are connected to the extremities of a similar 

 and parallel bar ^. Adjustable upon said bars op and m n is a 

 slider ?■ ;-', having a middle portion .j of insulating material, so that 

 the current from bar m n, for example, does not pass across said 

 slider r r' to bar op, but proceeds by the wire^ through the gal- 

 vanometer g". Suppose, now, that the telescopes z and i' are 

 sighted upon the distant object T, as before, and that the slider r 

 is at the middle point I of the parallel bars in n and op: the re- 

 sistances in the bridge will obviously not balance. It has already 

 been explained in connection with Fig. 5, how, by moving tele- 

 scope /'to the point H, the resistances might be balanced ; and if 

 that were done, with the arrangement shown in Fig. 6, the fact 

 would obviously be indicated by a deflection of the galvanomeler- 

 needle ; but now let it be assumed that the telescope i, after being 

 sighted upon the object T, is not moved, or, in other words, that 

 the observers respectively at the two telescopes / and i' simply 

 adjust their instruments in line with T. Obviously, then, the dis- 

 tance of the bridge from r to G (member a) is less than the dis- 

 tance from }■ to k (member b) by the length of the arc GH. Sim- 

 ilarly the distance on the bridge from r' to G (member c) is greater 



than the distance from r' to k (member d) by the length of arc 

 GH. 



Now let the resistance per unit length of the bars m n op be 

 made equal to or with some definite relation to the resistance per 

 unit length of the arcs h h\ and lay off on bar 711 ?i a distance r 3 

 and on bar ops. distance ?■' 3, said distances being such that the 

 resistance due thereto will be equal to that of the arc GH. Clearly, 

 if the end r of the slider be moved to the position 2 on bar 111 n, 

 the member a will be increased and the member b will be dimin- 

 ished by the distance r 2, which offers a resistance equal to one- 

 half that of arc GH ; and if the end r' of the slider be moved to 

 the position 2 on bar op, then the member c will be decreased and 

 the member d increased by the distance r' 2, which also has a re- 

 sistance equal to one-half of arc GH. As both ends of the slider 

 move simultaneously, it follows that when its extremities are 



JE^-" 



•i=<3) 



adjusted in the position 2, then the bridge will balance and the 

 galvanometer-needle will again be at zero. Applying this practi- 

 cally, let the bars in n p he laid off in suitable scale-divisions 

 from r to 11 and z' to p. The two telescopes z and i' being sighted 

 on the object, the distant observer watches the needle, and moves 

 the slider r r' along the bars m nop until it returns to zero. The 

 scale marked on the bars then shows an indication corresponding 

 to the length of arc GH, or, if desired, actual distances correspond- 

 ing to such indications. 



If the object be moving, the operation of determining its distance 

 is as easy as though it were stationary, and the indications are in- 

 stantaneous and continuous. With a 290-foot base-line on board 

 the " Chicago," one instrument being mounted in the bow and one 

 in the stern, the average error in the official trial before a board of 

 electrical and gunnery experts was less than six tenths of one per 

 cent. The set of instruments about being sent to the " Baltimore " 

 is expected to give still more accurate results. 



