Description of the Thunder-storm Alarum. 345 
The Thunder-storm Alarum ; as it serves toshow the effect which 
is produced by the passage of a charged cloud over an almosphe- 
rical electrometer. 
This instrument consists of a mahogany box about 6} inches 
in height, and about 3 inches in width, also 3 inches.in depth. 
The front (when not in use) is closed with a sliding piece of 
mahogany, like a sliding box lid. A glass tube A (Plate III. 
fig. 2,) is fixed at the top of the box, projecting some way into 
it; and through this a brass wire B passes, on the upper part 
of which is a brass ball C; and at the lower, a piece of brass D 
is screwed. A fine piece of flexible wire E (suchas is wound 
round cat-gut musical strings) is inserted into the ball C; a 
small bell F, on a brass pillar C, is fixed at the bottom of the 
apparatus. A brass ball G for a clapper, is suspended by a silken 
string from a wire H. The inside as well as the outside of the 
glass tube should be coated with melted sealing-wax or other 
insulating substance. 
There is no necessity for using an electrical machine, in order 
to make this instrument act: a piece of paper when dried by the 
fire is to be rubbed with India rubber (caoutchouc) which makes 
it highly electrical; the paper is then to be brought over the 
upper end of the flexible wire, in order to electrify the wire 
which passes through the glass tube; the clapper, which is sus- 
pended by the silken string, will then vibrate and strike the 
bell for some time, whilst the paper remains near the upper wire 
of the apparatus. The excited paper aptly represents an arti- 
ficial charged thunder cloud. 
A piece of woollen stuff (such as is made use of for gowns), 
when rubbed in the same way as paper, I think acts stronger. 
The above-mentioned wire being flexible, may be bent, if occa- 
sion should require it, so as to come near an electrified con- 
ductor, if a machine is used, or near a charged jar; but the prin- 
cipal use of its being flexible is, that there is not the chance of 
accidents occurring, which might happen from a stiff very sharp 
projecting wire. 
It is obvious that, if an atmospherical conducting rod were 
connected with this instrument, it might be used as a real 
thunder-storm alarum. ‘The ball C is perforated horizontally, 
through which a wire may be passed to suspend a pith-dall 
electrometer, or for other purposes. 
I am, sir, &c, 
Jan, 22, 1816, B. M. Forster. 
LXIX. On 
