The Observatory at St. Martin. 359 



throwing the light of a good lens upon them, the expansion is 

 easily depicted, and serves well for a night register. There is also 

 a Peltier's electrometer, another form of electrometer, consisting of 

 two gold leaves suspended to a rod of copper two feet long ; the 

 upper end being furnished with a wire box, in which is kept burL- 

 ing some rotten wood (touch-wood). 



The Anemometer consists of a direction shaft and a velocity 

 shaft : to the top of the direction shaft is placed the vane, which 

 is eighteen feet in length. The shaft is made of three pieces, to 

 insure lightness and more easy motion : each piece is connected 

 by means of small iron-toothed wheels. The two shafts are six 

 feet apart, and work on cross-arms from a mast firmly fixed in the 

 ground. The vane passes some six or eight feet above the velocity 

 shaft, and does not in any way interfere with the other movements. 

 The lower extremity of these shafts are all furnished with steel 

 points, which work on an iron plate or a piece of flint, and pass 

 through the roof of the Observatory ; the openings being protect- 

 ed by tin parapluies fixed to the shaft, and revolving with tbem. 

 Near the lower extremity is placed a toothed-wheel, eight inches 

 in diameter, connected to another wheel of the same diameter^ 

 which carries upon its axis a wooden disc, thirteen inches in dia- 

 meter, upon which is clamped a paper register (old newspapers 

 answer very well) washed over with whiting and flour paste. Upon 

 the surface of this register is traced by a pencil the direction of 

 the wind. This register is renewed every twelve hours. 



The velocity shaft is in two pieces, connected by means of the 

 toothed wheels and steel pivots, as in the direction shaft ; and, 

 practically, the friction is nil. At the top of the velocity shaft 

 are fixed three hemispherical tin or copper caps, ten inches in dia- 

 meter, similar in construction to those of the Rev. 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 is 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, 2£ inches in diameter ; this moves a second, 

 or ten-toothed, wheel, which also gives movement to a third wheel. 

 This marks a hundred revolutions of the caps, which are so cal- 

 culated that each one hundred revolutions are equal to one mile 

 linear; and whenever one hundred revolutions have been accom- 

 plished, a small lever is elevated by means of an inclined plane, 



