1863.] 685 



occasions by the heat of a spirit-lamp, I determined to dispense, if 

 possible, with water-dropping, and collect the electricity of the air by 

 a burning match. Accordingly, from Dec. 2nd to March 30th I 

 employed matches such as are used with Professor Thomson's Por- 

 table Electrometer (consisting of rolls of blotting-paper prepared with 

 nitrate of lead), the burning match being fixed on the end of the 

 water- dropping pipe. The effect of shortening the pipe is to weaken 

 the electricity collected, in a constant ratio, which I have ascertained, 

 by observations taken alternately with the pipe shortened and re- 

 stored to its full length, .to be about 1 to 3-1. This difference has 

 been allowed for in reducing the observations, as it was very desirable 

 to furnish my tabulated results in a form easily admitting of com- 

 parison by inspection. I have, nevertheless, inserted the letter S in 

 the column " Remarks " against all observations taken with pipe 

 shortened. 



I have in like manner compared, by alternate observations, the 

 results obtained respectively by water- dropping and burning match, 

 and my observations lead to the inference that the results obtained 

 by both methods are the same. The letters M and "W, when they 

 occur in the column " Remarks," denote respectively that the ob- 

 servations against which they stand were taken with burning match 

 and water-dropping. (See Table IV.) 



4. For testing and measuring the electricity thus collected I have 

 used the "Station Electrometer," except in a few cases, when I 

 have employed the "Portable Electrometer." Both instruments 

 are inventions of Professor Thomson. The former consists of a 

 Leyden jar, having within it a needle of aluminum suspended by a 

 glass fibre, and connected (by platinum wires dipping in sulphuric 

 acid) with the inner coating of the jar. In the neighbourhood of 

 the needle are two brass plates, also connected with the inner coating 

 of the jar, called the " repelling plates," because their function is to 

 repel the two ends of the needle, causing it to rotate, and thereby 

 twist the glass fibre. A brass cage surrounds both the needle and 

 the repelling plates. It hangs from glass pillars, which insulate it 

 from the jar (and therefore also from needle and repelling plates) ; 

 and an arm of brass attached to it projects through a hole to the 

 outside of the jar, furnishing the means of connecting the cage with 



VOL. XII. 3 C 



