354 
DE. EVEEETT ON ATMOSPHERIC ELECTRICITY. 
The use of a gauge-electrometer has been superseded by the introduction of a micro- 
meter-screw attached to one of the four segments, and regulating its distance from the 
others. By diminishing or increasing this distance the sensibility of the instrument can 
be increased or diminished at pleasure; and by attending to this adjustment as often as 
may be necessary (say, once a day), the sensibility can be kept practically constant in 
spite of variations in the charge of the Leyden jar*. 
The retention of charge by the jar has been greatly improved by closing up the space 
round the open electrode (as T, fig. 1) with vulcanite. 
The portable electrometer employed in some of my Windsor observations has been 
superseded by a smaller and at the same time more sensitive instrument, in which the 
distance between two parallel plates, one of which is connected with a charged Leyden 
jar and the other with the conductor to be tested, is varied at pleasure by a micrometer- 
screw so as to obtain a constant force of attraction between them. Equal differences of 
potential, whether in the conductor tested or in the charge of the jar, correspond to 
equal movements of the micrometer-screw, and the potential of the conductor can thus, 
by comparison with an earth-reading, be found by mere addition and subtraction. 
The curves which have been measured and reduced comprise the two years commencing 
with June 1862 and ending with May 1864. The method of procedure was as follows :-r— 
1. Ordinates were erected at every hour and half hour, careful attention being paid to 
the times of commencing and ending, which were in every case indicated by entries made 
on the photographic sheets by the Kew observers. In placing the ordinates, it was 
assumed that each sheet had moved with uniform velocity through its whole length, 
but it was not assumed that different sheets had moved with the same velocity ; in fact 
the difference in the lengths of the curves, taken in connexion with the times of begin- 
ning and ending, showed that the velocities of different sheets must have differed by (in 
extreme instances) about 5 per cent. As it was impossible therefore to use one time- 
scale for all the curves, about twenty different time-scales were prepared differing by 
small gradations one from another, and for the measurement of each curve that scale 
was employed which suited it best. 
2. The ordinates erected at the half-hours {e.g. half-past one, half-past two, &c.) were 
joined by straight lines drawn by hand in such a manner as to give and take equal areas 
as nearly as the draughtsman could judge. 
3. The lengths intercepted by these joining lines on the hourly ordinates were mea- 
sured with a scale divided to millimetres, and were adopted as the mean heights of the 
curve for each hour. 
Whenever the curve for part of an hour was not traceable, a blank was left for that 
hour, and whenever the curve was partly above and partly below the zero-line (showing 
that the electricity was partly positive and partly negative), the algebraic mean was taken. 
The measurements made in the manner above described were entered in a book in 
* Still more recently a “ replenisher ” acting by induction and convection has been added, by means of which 
the jar can easily be kept at a nearly constant charge. 
