358 The Observatory at St. Martin. 



the accumulated electricity to the receiver which is placed in the 

 observatory. 



The receiver consists of a cross of brass tube (gas tubes), each 

 about 2 feet long, and is screwed into a large tube fitting upon a 

 glass cone, which is hollow, forming a system of hollow pipes for 

 the passage of the heat internally, and keeping up a certain amount 

 of dryness and consequent insulation. The glass cone is fixed 

 upon a table over an opening made in it, fitting to the hollow part 

 of the cone. Immediately under this table is placed a small stove 

 of sheet-iron, about 8 inches in diameter, made double, the space 

 of about 1 inch being left between the two chambers ; and this 

 plan has been found to effect a good insulation by keeping the 

 whole of the apparatus warm and dry. Charcoal is used as fuel, 

 and is, I think, preferable to a lamp. A coating of suet or tallow 

 is applied to the glass cones or pillars. Care must be taken not 

 to rub or polish the collecting apparatus as it seems to deteriorate 

 its power of collecting and retaining atmospheric electricity ; and 

 I have found that its collecting powers increase with its age. 

 Suspended from these cross arms hang the electrometers. 1. 

 Bennefs electroscope of gold leaves ; this scarcely needs a descrip- 

 tion. 2. Voltds electrometers, No. 1, consisting of two straws, 

 two French inches long : a very fine copper wire passes through 

 these straws, which are suspended from the cross-arms. This 

 electrometer is furnished with an ivory scale, the old French inch 

 being divided into twenty-four parts, each being 1°; this forms 

 the standard scale for the amount of tension. 2. Volta's electro- 

 meter, No. 2, is similar to the No. 1, but the straws are five times 

 the weight of No. 1, so that one degree of Volta's No. 2 is equal to 

 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 Distinguishes 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 



