114 Vi REPORT—1849. 
a part of a regular diurnal movement, or have occurred at such hours as are 
generally distinguished by exhibiting an increase of tension. It was con- 
sequently considered that a rejection of them would to a certain extent in- 
terfere with the development of the diurnal and annual curves. The values 
in the table, as well as throughout the discussion, are recorded in terms of 
Volta’s electrometer No. 1. The observations were taken with Henley’s in- 
strument, ] degree of which has been approximately considered to be equal 
to 100 divisions of Volta No. 1*. 
* On the 13th and 14th of July 1849, the reporter attended at the observatory for the 
purpose of comparing the electrometers, and especially determining the value of the readings 
of Henley’s electrometer in terms of Volta’s standard No.1. The following are the results of 
the comparison. It appears from upwards of two hundred readings, the charge varying be- 
tween 50 diy. and 110 div. of Volta No. 1 as read from the scale of the No. 2 electrometer, 
that the mode of reading adopted by the observer at Kew, during the five years, was to bring 
the eye into such a position that the inner edge of the straw should coincide with the division 
read on the ivory are of the instrument. By this mode of reading, I° of Henley would very 
nearly equal in value 100 div. of Volta; this value has accordingly been retained, as most in 
accordance with the mode of reading adopted. It will be however evident that the true read- 
ing would be given, not by either edge of the straw, but by the centre: the diameter of the 
_ Straw is equal to 2°; consequently when the inner edge coincides with 1°, the true reading must 
be 2°. From this it is clear that the values in the following discussion are relatively too high, 
but they will not interfere with the results further than by eapanding the curves ; the inflexions, 
points of maxima and minima, and the general form of the curves, will be the same, conse- 
quently the results derived from these curves will be unaffected. It would have been desirable 
to have applied a correction for this difference in the mode of reading, had not a greater dif- 
ficulty presented itself in the dissimilarity of the construction of the two instruments, by which 
the values at different parts of the scale of Henley’s instrument acquire different values in terms 
of Volta’s instrument. The small extent of range common to both instruments renders it very 
difficult to express the higher readings of Henley at all accurately in terms of Volta. It is 
therefore considered best under the circumstances to retain the values as given in the tables, 
especially as the results are not materially interfered with ; and endeavcur to point out a mode 
by which the readings of Henley’s instrument may be accurately expressed in terms of Volta, 
as well as to indicate a more precise method of observation. 
The standard electrometer No. | of Volta is so constructed that a given electric force causes 
a pair of straws of a known weight to diverge. Their divergence is measured on a circular arc 
of the same radius as the length of the straws, which is so graduated as to indicate half the 
distance in arc between the extremities of the straws in half-Parisian lines, each of the divi- 
sions, which are at equal distances from each other, being equal to halfaline. It is clear from 
this construction of instrument, that upon measuring the distance between the straws ina 
right line, the sine of half the angle subtended by the extremities of the straws is proportional 
to the electric tension of the charge. : 
The electrometer No, 2 is so constructed that each division is exactly equal to five of No. 1, 
and the circular arc is graduated to read at once the electric tension in terms of No. 1. The 
difference in the electrometers consists in the straws of No. 2 being heavier than those of No. 1, 
in such a proportion as to increase the value of the readings in the ratio above mentioned. Asin 
No. 1 the sine of half the angle of divergence is proportional to the tension, soin No, 2 precisely 
the same value of the tension obtains, viz. the sine of half the angle of divergence, the linear 
value of the sine itself being proportional to its value in No. 1 for the same force: thus, a force 
that would diverge the straws in No. 1 to an angle of 30° would only open them in No. 2 to an 
angle of 6°, and in each instrument the sines of 15° and 3° respectively would represent the 
same force. There is however no necessity to employ such a determination of the force, the 
graduation of each instrument being amply sufficient for the purpose. 
The Henley’s electrometer is so constructed that the force is measured by a straw termi-~ 
nating in a pith-ball, which together constitute a pendulum that is inserted in a ball working 
by two fine steel pivots. This pendulum diverges by the electric force from the perpendicular, 
the angular amount of divergence being measured by a quadrant, graduated to degrees of the 
circle on an ivory scale. As it is thus used, the readings are not very readily comparable with 
those of the Volta’s electrometers, in consequence of the Henley readings being in arc, while 
those of Volta are in linear measure. This difficulty may however be readily overcome by 
immediately measuring the sine of the angle of divergence, which in this instrument is a 
measure of the electric tension. Nothing further would be required than to place the elec- 
trometer in a convenient position for observation by a theodolite, which should be firmly fixed 
at a known distance from it, The centre of the azimuth circle should be in the precise vertis 

