12 REPORT—1849, 
This has arisen from the want of a simple and efficient instrument by which such 
observations could be made. The most valuable results which have hitherto been 
obtained have been made with fixed electric apparatus. That established at the 
Observatory of the British Association at Kew, under the superintendence of Mr. 
Ronalds, and in which he has introduced so many important improvements which 
render it, in perfectness of insulation, and the comparability of its attached electro- 
meters, superior to any hitherto erected, will no doubt, when the observations 
made at the establishment during the past five years are reduced and discussed, as 
is now being done by Mr. Birt, yield valuable results. Still such apparatus are 
too costly, and require too many precautions in their establishment and manipula- 
tion to be recommended for general use. 
Meteorologists will therefore learn with satisfaction that this deficiency is now 
supplied by the late M. Peltier’s induction electrometer, a portable instrument, 
simple in its construction, certain in its results, and of which any number may be 
made perfectly comparable with each other. One of these instruments is on the 
table. A hollow ball of copper, four inches in diameter, is placed at the top of a 
rod of the same metal, which is terminated at its lower extremity by a much smaller 
ball. From the last-mentioned ball, insulated from the glass cover by a lump of 
shell-lac, descends a copper rod, which bifurcates and forms a kindof ring. At the 
centre of this ring a small copper needle, which forms the essential part of the in- 
strument, moves freely, balanced on a pivot. When the electrometer is in its na- 
tural state, the needle is brought to the magnetic meridian by a much smaller mag- 
netic needle which is parallel to it, and fixed immediately above it. Another copper 
needle, much thicker than the moveable one, forms a system with the rod which 
~ descends into a glass tube filled with shell-lac and fixed into the wooden stand. 
Thus the entire metallic part of the apparatus is insulated, and electricity can be 
communicated from it neither to the glass cover nor the stand. This insulation 
must be established with the greatest care. The stand is furnished with three level- 
ing screws, which enables it to be placed horizontally. To prepare the instrument 
for an observation, it must be so placed that the fixed needle shall be in the direc- 
tion of the magnetic meridian. In this position, the moveable needle, directed by 
its small magnetic needle, places itself parallel to the fixed needle. If now a body 
electrified, positively or negatively, be held above the ball, it decomposes by induc- 
tion the electricity of this ball and its metallic appendages. If the body be posi- 
tively electrified, at the upper extremity of the ball the negative electricity is coerced 
by the positive electricity in presence, while in the lower part of the instrument the 
free positive electricity causes the small needle to diverge from the position which it 
had at first, and its angle of deviation from the fixed needle will be greater as the 
free electricity is more considerable. The angle of deviation is read off by means of 
two graduated circles, one of which is pasted to the stand and the other to the _ 
glass cover ; by this parallax is avoided in the readings. If while the ball is in- 
fluenced by the external electricity, the stem is touched, the free electricity, which 
we willassume to be positive, will be removed and the needle will replace itself in the 
magnetic meridian. Ifthe inducing body which coerces the negative electricity at 
the upper part of the ball be removed immediately after, this electricity will become 
free, and the moveable needle will diverge anew. 
I will state in M. Peltier’s own words the mode of operating with this instrument 
when the electricity of the air is to be observed. ‘‘ When I wish to ascertain the 
electric tension accumulated in the atmosphere, I ascend to the terrace, I place the 
instrument on a stand raised about 6 feet, I put it in equilibrium by touching the 
lower part of the stem, I then descend, and place the instrument on the table ap- 
propriated to it: all this is performed with great rapidity, and requires only eight 
seconds: when the instrument is put in equilibrium, the arm of the observer must 
be raised as little as possible, for if it be raised sufficiently to reach the globe, the 
hand becoming negative by induction will repel the negative electricity of the ball; 
it will neutralize the positive portion which will be attracted towards it, and the 
instrument will be charged negatively at the moment of the removal of the hand. 
The stem must therefore be touched as low as possible, and with as slender a body 
as a metallic wire, in order to avoid the inductive action of the mass of the hand 
upon the remainder of the stem. The equilibrium being established when it is 

