Nov. 27, 1879] 



NATURE 



95 



the ground. A heliostat at H reflected the sun's rays through 

 the slit at S to the revolving minor, R, thence through a hole in 

 the shutter, through the lens, and to the distant mirror. The 

 interior of the building was painted black. In a room under- 

 neath the part, R, was the apparatus which supplied the air for 

 turning the mirror. 



The heliostat was kindly furnished by Dr. Woodward, of the 

 Army Medical Museum, and was a modification of Foucault's 



form, designed by Keith. It was found to be easy of adjust- 

 ment and quite accurate. 



The light was reflected from the heliostat to a plane mirror, 

 M, Fig. 3, so that the former need not be disturbed after being 

 once adjusted. 



The revolving mirror was made by Fauth & Co., of Washington. 

 It consists of a cast-iron frame, Fig. 4, resting on three levelling 



Clark, about one and a quarter inch in diameter and o - 2 inch 

 thick. It was silvered on one side, the reflection taking place 

 from the silvered side. A species of turbine wheel is held 

 on the axle by friction. This wheel has six openings for the 

 escape of air. The air entering on one side acquires a rotary 

 motion in the box, carrying the wheel with it, and this motion is 

 assisted by the reaction of the air in escaping. The disc above 

 the mirror serves the purpose of bringing the centre of gravity in 

 the axis of rotation. This was done, following Fc ucault's plan, by 

 allowing the pivots to rest on two inclined planes of glass, allowing 

 (he arrangement to come to rest, and filing away the lowest part of 

 the disc; then trying again, and so on, till it would rest in in- 

 different equilibrium. The part corresponding to the disc in 

 Foucault's apparatus was furnished with three vertical screws, by 

 moving which the axis of figure was brought into coincidence with 

 the axis of rotation. This adjustment was very troublesome, and in 

 this apparatus was found unnecessary. When the adjustment is 

 perfect the apparatus revolves without giving any sound, and 

 when this is attained the motion is regular and the speed great. 

 A slight deviation causes a sound, due to the rattling of the 

 pivots in the sockets, the speed is very much diminished, and 

 the pivots commence to wear. In Foucault's apparatus oil was 

 furnished to the pivots, through small holes running through the 

 screws, by pressure of a column of mercury. In this apparatus 

 it was found sufficient, at high speed, to touch the pivots occa- 

 sionally with a drop of oil. 

 The quantity of air entering could be regulated by a valve, 



screws, one of which was connected by cords to the table at 

 S, Fig. 3, so that the mirror could be inclined forward or back- 

 ward while making the observations. Two binding screws, 

 terminating in hardened steel conical sockets, hold the re- 

 volving part. This consists of a steel axle, the pivots 

 being hardened, expanding into a ring, which holds the 

 mirror. The latter was a disc of plane glass, made by Alvan 



to which was attached a cord leading to the table. The revolving 

 mirror was mounted on a brick pier. 



The apparatus for measuring the deflection was made by 

 Grunow, of New York. 



It consists of a screw, with divided circle, Fig. 5- To the 

 frame is attached an adjustable slit. On the screw travels a 

 carriage which supports the eyepiece, which consists of an 

 achromatic lens, having in its focus at the end of the tube a 

 single vertical silk fibre. 



Next the eye is a piece of plane glass, inclined at an angle of 

 45°. In measuring the deflection, the eyepiece is moved till the 

 cross hair bisects the slit (with w hich it is nearly in the same 

 plane), and the reading.of the scale and divided circle gives the 

 position. This is made once for all, unless the slit be altered in 

 width or position. Then the eyepiece is moved till the cross 

 hair bisects the deflected image of the slit ; the reading of 

 scale and circle are again taken, and the difference in readings 

 gives the deflection. The screw has no lost motion, so that 

 readings may be taken with the screw turned in either direction. 

 This apparatus is mounted on a standard with rack and pinion, 

 and the base furnished with levelling screws. 



To regulate and measure the speed of rotation, a tuning fork, 

 bearing on one prong a steel mirror, was employed. This was 

 kept in vibration by a current of electricity from five gravity 

 cells. The fork was so placed that the light from the revolving 

 mirror was reflected to the piece of plane glass in the eyepiece, 

 thence reflected into the eye. When fork and revolving mirror 

 are both at rest, the eye sees an image of the revolving mirror. 



