September 17, 1886.] 



SCIENCE. 



257 



at the telephone. The sound, like a tapping on the 

 diaphragm of the telephone, came clear and distinct ; 

 and, when the captain turned the half-minute glass, 

 I had no difficulty in counting the revolutions of the 

 shaft, seventy-four in number. On referring to the 

 table, it appeared that we were making eight and 

 five-tenths knots, which was the exact speed of the 

 ship as shown also by the revolutions of the engine. 

 The experiment was very satisfactory and extremely 

 interesting. Captain Bretel wrote out a description 



of his invention, with drawings, which he gave to 

 me. 



In the accompanying diagram, A, B, C, D, are four 

 hemispherical cups on the ends of four arms affixed 

 to a brass shaft, E F, which revolves freely in lig- 

 num-vitae bearings at the extremities of the jaw 

 H K L. For convenience in removing the shaft, the 

 bearing at His detachable, being secured in place by 

 the wing-nut I. At one end of the shaft is a small 

 lignum-vitae pulley, P Q, having on one side a strip 

 of copper, mm', fastened in position by screws pass- 



ing through the shaft, which screws make electrical 

 connection between the copper strip and the shaft. 

 An elastic tongue, O, on the screw N, bears upon the 

 other end of the shaft, giving metallic contact 

 between shaft and jaw. The log is towed some dis- 

 tance astern of the vessel by a log-rope, which is 

 attached to the jaw by a copper shank and a clamp- 

 ing-screw, T. To this copper shank is secured a strip 

 of zinc, thus forming a weak galvanic battery. 

 An insulated wire furnishes electrical connection 



between the commutator brush or tongue, E S, and a 

 Bell telephone in the chart-room of the ship, the 

 wire making several spiral turns around the log-rope 

 to guard against snarling. With the log thus towing 

 astern of the vessel, it is evident that at every revo- 

 lution of the log-wheel A B C D an electric circuit 

 between the telephone and tbe zinc and copper of the 

 shank is made and broken by means of the copper 

 strip mm' of the lignum-vitae roller coming into con- 

 tact with the elastic tongue R 8, each ' make and 

 break ' being signalled by a tick of the telephone. To 



