290 THE OBSERVATORY AT ST. MARTIN, ISLE JESUS, C. E. 



three heini spherical tin or copper caps, ten inches in diameter, simi- 

 lar in construction to those of the Eev. Dr. Eobinson of Armagh, 

 and are firmly rivetted to three iron arms of f-inch iron. These 

 caps revolve always in the same direction, and one revolution ia 

 found to be just one-third of the linear velocity of the wind. I 

 have no reason to doubt Dr. Robinson's formula for this calculation. 

 At the lower extremity of the velocity shaft is fixed a one-toothed 

 wheel, 2f inches in diameter ; this moves a second, or ten-toothed, 

 wheel, which also gives movement to a third wheel. This marts a 

 hundred revolutions of the caps, which are so calculated that each 

 one hundred revolutions are equal to one mile linear ; and whenever 

 one hundred revolutions have been accomplished, a small lever 

 is elevated by means of an inclined plane, 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, upon 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 hour, and the lever before 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 direc- 

 tion of the wind, the moment of its changes, and the point from 

 which it veered. The extreme 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 serves 

 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. 



