THE OBSERVATORY AT ST. MARTIN, ISLE JESUS, C. E. 289 



five of No. 1. Henly's electrometer is a straw suspended and fur- 

 nished with a small pith ball : each of the degrees of Henly's is 

 equal to 100° of No. 1 of Volta's. These electrometers are all sus- 

 pended from the cross-arms. A discharging apparatus, furnished 

 with a long glass handle, measures the length of the spark, and serves 

 also as a conductor to carry the electricity collected to the earth, 

 and is also connected by a chain and iron rod passing outside of the 

 observatory for about twenty yards, and buried under ground. 



Various forms of DistinguisJiers are used to distinguish the kinds 

 of electricity. The Volta's electrometers may be rendered self- 

 registering, with great facility, by the photographic process. By 

 placing a piece of the photographic paper behind the straws, and 

 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 Pel- 

 tier's electrometer, and 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 burning 

 some rotten wood (touch-wood.) 



The Anemometer consists of a direction shaft and a velocity shaft z 

 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 Observa- 

 tory ; the openings being protected by tin parapluies fixed to the 

 shaft, and revolving with them. 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 diameter, 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, prac- 

 tically, the friction is nil. At the top of the velocity shaft are fixed 



