360 The Observatory at St. Martin. 



fixed upon the edge of the last wheel, and which gives motion to 

 the lever. The other extremity of the lever is furnished with a 

 fine steel point, which dots off, npon a paper register, the miles as 

 they pass. This register is of paper, one and a quarter inch wide, 

 and is removed every twelve hours. 



Between the two shafts, at the lower extremities, are placed two 

 runners of wood, rebated, to receive a slide or train, which carries 

 the register. To the underside of this slide is fixed a rack, and it 

 is moved by a pinion, the movement of which is communicated 

 by a clock, — the cord of the weight being passed over a wheel 

 and pulley, — and advances one inch per honr, and the lever befoie 

 described dots off the miles as the register advances under the steel 

 point. In this manner it shows the increase and decrease of the 

 velocity, and also the moment of its change. Attached to this 

 moveable train is a rod of wood carrying a pencil, which passes 

 over the disc connected with the direction shaft, and there traces, 

 as it advances, the direction of the wind, the moment of its changes, 

 and the point from which it veered. The extrema height of the 

 vane is forty feet, but this might be increased if required. The 

 clock is wound up every twelve hours, which brings back the train 

 to its starting point. 



There are also a polariscope, prisms, and glasses of different co- 

 lors, for experimenting on the different rays of light, in connexion 

 with the germination of seeds, and the art of photography. The 

 Observatory possesses a quadrant and artificial horizon, which 

 serve for measuring the diameter of halves, and altitudes of auroral 

 arches, &c. : also a dial for the indication of the direction and 

 course of the clouds ; and other minor instruments. 



