motion. of the cloud, and the quantity of 
the electricity contained in it. If a small 
cloud hangs suspended under * large one 
loaded with electric matter, pointed conduct- 
ors on a building underneath will receive 
the discharge by explosion, in preference 
to those 1 terminated by balls ; the small 
cloud forming an interruption, which allows 
only an instant of time for the discharge. 
Earl Stanhope has communicated to the 
public, in a treatise on this subject, some 
essentials to be observed in the erection of 
conductors to buildings : he advises, that the 
upper fifteen or twenty inches of the rod 
should be composed ot' copper and not of 
iron, as the latter being exposed to the wea- 
ther will rust, and rust does not conduct 
electricity ; that the iron part of the rod 
should be painted, but not the upper part of 
it, because paint is no conductor. He fur- 
ther advises, that the upper extremity of a 
conducting rod should not only be accurate- 
ly pointed and finely tapered, but that it 
should be extremely prominent, about ten 
or fifteen feet above all the parts of the build- 
ing. which are the nearest it. It may be 
added, that a conductor should always be 
carried in the earth some feet beyond the 
foundation of the building, and should, if 
possible, terminate in water. 
The safest situation during a thunder 
storm is the cellar ; for when a person is be- 
low' the surface of tire earth, the lightning 
must strike it before it can reach him, and 
will of course, in all probability, be expend- 
ed on it. Dr, Franklin advises persons ap- 
prehensive of lightning to sit in the middle 
of a room, not under a metal lustre or any 
other conductor, and to lay their feet up 
upon another chair. It will be still safer, 
Ire adds, to lay two or three beds or matrass- 
es in the middle of the room, and folding 
them double, to place the chairs upon them. 
A hammock suspended by silk cords would 
be an improvement upon this apparatus. 
Persons in fields should prefer the open parts 
to the vicinity of trees, &c. The distance 
of a thunder storm, and consequently the 
danger, are not difficult tube estimated. As 
/• • light travels at the rate of 72,420 leagues 
in a spcond of time, its effects may be con- 
sidered as instantaneous within any moderate 
distance. Sound, on the contrary, is trans- 
mitted only at the rate of 1,142 feet, or 
about 380 yards in a second. By accurately 
observing therefore the time winch inter- 
venes between the flash and the noise of 
thunder which follows it, a very near calcu- 
lation may be made of its distance, and we 
know no better means of removing unne- 
cessary apprehensions. 
The success of Dr. Franklin, in ascertain- 
ing the cause of thunder and lightning, in- 
duced succeeding philosophers to apply the 
same theory to the explanation of the other 
atmospherical phenomena. From a number 
of observations, the indefatigable Beccaria 
I endeavours to account for the rising of va- 
pours and the fall of rain, upon electrical 
principles ; and it must be confessed, that 
if it is not a primary agent in these effects, 
it would be rashness entirely to deny its in- 
fluence. This philosopher supposes, that 
previously to rain a quantity of electric matter 
escapes from the earth, and in its ascent to 
the higher regions of th« air collects and 
ELECTRICITY. 
conducts into its path a great quantity of 
vapours. The same cause that collects will 
condense them more and more, till in the 
places of the nearest intervals they come al- 
most into contact, so as to form small drops, 
which, uniting with others as they fall, come 
down in rain. The rain he supposes to fall 
heavier in proportion as the electricity is 
more vigorous. 
Hail lie supposes to be formed in the high- 
er regions ot air, where the cold is intense, 
and where the electric matter is very co- 
pious. lii these circumstances, a great num- 
ber of particles of water are brought near 
together, v'here they are frozen, and in their 
descent collect other particles ; so that the 
density of the substance of the hailstone 
grows less and less from the centre, this being 
formed first in the higher regions, and the 
surface being collected in the lower. Agree- 
ably to this, it is observed, that on moun- 
tains, hailstones as well as drops of rain are 
very small, there being but a small space 
through which they can fall. 
Clouds of snow differ in nothing from 
clouds of rain, but in the circumstance of the 
cold which freezes them. Both the regular 
diffusion of snow, and the regularity of the 
parts of which it consists, shew the clouds of 
snow to be actuated by some uniform cause 
like electricity. 
Consistent with this theory is the fact, that 
vapours never rise to a great height without 
producing meteors. Almost all volcanic 
eruptions are accompanied with lightning. 
The column of vapour, which proceeds from 
the bow els of a volcano, is continually trans- 
versed by lightning, which sometimes seems 
to proceed from the higher regions, some- 
times from the column itself. These light- 
nings were observed by the younger Pliny, 
in the eruption which killed his uncle; and 
sir William Hamilton has observed them se- 
veral times. The aurora borealis is also ge- 
nerally supposed to be electrical ; its light 
seems to be produced by the electric fluid, 
while it is condensed in passing in the co- 
lumns of elevated vapour. 
Mr. Adams’s description of this meteor, in 
his Lectures, is as follows : The appearances 
of the aurora come under four different de- 
scriptions. 1st, A horizontal light, like the 
morning aurora, or break of day. 2dly, 
Fine, slender, luminous beams, well defined, 
and of dense light. These often continue a 
quarter, a half, or a whole minute, ap- 
parently at rest, but oftener with a quick 
lateral motion. 3dly, Flashes pointing up- 
ward, or in the same direction with the 
beams, which they always succeed. These 
are only momentary, and have no lateral 
motion ; but they are generally repeated 
many times in a minute. They appear 
much broader, more diffuse, and of a weaker 
light, than the beams : they grow gradually 
fainter till they disappear; and sometimes 
continue for hours, flashing at intervals. 
4thlv, Arches, nearly in the form of a rain- 
bow’: these, when complete, go quite across 
the heavens, from one point of the horizon to 
the opposite point. 
When an aurora happens, these appear- 
ances seem to succeed each other in the fol- 
lowing order: 1, the faint rainbow-like 
arches ; 2, the beams ; and, 3, the flashes. 
As for the northern horizontal light, it ap- 
607 
pears to consist of an abundance of flashes 
or beams blended together by the situation 
of the observer. 
The beams of the aurora borealis appear at 
all places to be arches of great circles of the 
sphere, with the eye in the centre ; and these 
arches, if prolonged upwards, would all meet 
at one point. 
The rainbow-like arches all cross the mag- 
netic meridian at right angles. When two 
or more appear at once, they are concentric* 
and tend to the east and west ; also the 
broad arch of the horizontal fight tends to 
the magnetic east and west, and is bisected by 
the magnetic meridian ; and when the aurora 
extends over any part of the hemisphere, whe- 
ther great or small, the line separating the il- 
luminated part of the hemisphere from the 
clear part, is half the circumference of a 
great circle crossing the magnetic meridian 
at right angles, and terminating in the east 
and west: moreover, the beams perpendicu- 
lar to the horizon, are only those on the mag- 
netic meridian. 
That point in the heavens to which the 
beams of the aurora appear to converge, at 
any place, is the same as that to which the 
south pole of the dipping-needle points at 
that place. 
The beams appear to rise above each 
other in succession ; so that of any two 
beams, that which has the higher base has 
also the higher summit. 
Every beam appears broadest at or near 
the base, and to grow narrower as it ascends ; 
so that the continuation of the bounding lines 
would meet in the common centre to which 
the beam tends. 
The height of the rainbow-like arches of 
the aurora is estimated by Mr. Dalton to 
be above the earth’s surface about 130 Eng- 
lish miles. 
A very beautiful experiment will illustrate 
this appearance in the heavens : Fig. 28 is 
called the luminous conductor. A is a glass 
tube about two feet long, capped at both 
ends with brass, having one of the ends fur- 
nished with a stop-cock, and a screw’ to tit 
into the plate of the air-pump. This tube 
is exhausted of air, and when it is placed in 
the circuit of the electric fluid, by fixing a 
chain to each end, which is connected with 
the positive and negative parts of the ma- 
chine, the electricity in passing through it, 
exhibits a beautiful luminous appearance, 
very much resembling the aurora borealis. 
See Aurora Borealis. 
It was intimated that water-spouts are 
among the phenomena which some philoso- 
phers have attempted to explain on electric- 
al principles. A w'ater-spout is a most for- 
midable phenomenon, and is indeed capable 
of causing great ravages. It commonly be- 
gins by a cloud, which appears very small, 
and which mariners call the squall ; which, 
augments in a little time into an enormous 
cloud of a cylindrical form, or that of a re- 
versed cone, and produces a noise like au 
agitated sea ; sometimes emitting thunder and 
lightning, and also pouring down large quan- 
tities of rain or hail, sufficient to inundate 
large vessels, overset trees and houses, and 
every thing which opposes its violent impe- 
tuosity. 
These water-spouts are more frequent at 
sea than by land ; and sailors are so convin- 
ced of their dangerous consequences, that 
2 
