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_Thunder and Lightning._
Conductors affixed to houses should always be pointed, and the point
should be kept in a state of cleanliness, and the conductors should
terminate in a moist stratum of earth, or in London it might safely be
conveyed into the common sewer. It has been objected to the use of
pointed conductors, that we invite the lightning to the point; and that
is true to a certain extent, and in gunpowder mills the conductor should
be placed at some distance from the building. The conducting rod should
be of copper or iron, and from half to three-fourths of an inch in
diameter, so as not to be readily forced. Its upper end should be
elevated about three or four feet above the highest part of the
building, and all the metallic parts of the roof should be connected
with the rod, which should be continuous throughout. As regards the
question of what is the safest situation in a thunder-storm, we should
be pretty safe in the middle of a large room in bed; we should be pretty
safe among the feathers, which are bad conductors; but as the bell-wires
will conduct the electricity into the room, the bed should be removed
from them. It would be well to stand at a distance from the chimney on
a woollen rug, which is a non-conductor. When out of doors, I scarcely
need to say, that you should never stand under a tree; the tree being
moist, the electric fluid generally passes down between the bark and the
substance of the tree, splitting it in all directions, and the lightning
will pass to the best conductor near it; if any unfortunate animal
should happen to be under the tree, it will be killed. The safest plan
is to go toward the middle of the field, at a distance from any tree,
and to stretch yourself out upon the ground, although this is not a very
pleasant situation, especially in hard rain. During a thunder-storm, the
earth is in a state of electricity as well as the clouds, and the light
and heat which are produced at the explosion indicate the annihilation
of the two electricities. Sometimes the discharge is only from cloud to
cloud, sometimes from the earth to the clouds, and sometimes from the
clouds to the earth, as one or other may be in the positive or negative
state. The clouds are usually more or less electrical when the vapour,
floating about in the atmosphere, is condensed, and the earth being
brought into an opposite state of electricity by induction, a discharge
takes place, when the clouds approach within a certain distance, and
sometimes the electric cloud perches upon a hill, and then discharges
itself. The electricity passes through the clouds in a zig-zag
direction, and the undulation of the air which it produces is the cause
of the noise which we hear, called thunder, which is more or less
intense, and of longer or shorter duration, according to the quantity of
air acted upon, and the distance of the place where the report is heard
from the point of discharge. If the danger be great, we have seldom any
opportunity to count the time which elapses between the appearance of
the lightning and the report: electrical effects take place at no
sensible time; it has been found, that a discharge through a circuit of
four miles is instantaneous, whilst sound moves at the rate of about
twelve miles in a minute. So that, supposing the lightning to pass
through a space of some miles, the explosion will be first heard from
the point of the air agitated nearest to the spectator; it will
gradually come from the more remote parts of the course of the
electricity, and, last of all, will be heard from the very extremity;
and the different degrees of the agitation of the air, and the
difference of the distance, will account for the different intensities
of the sound, and its apparent reverberations and changes. If you can
count from two to three seconds between the appearance of the lightning
and the sound, there is seldom much danger; and when the interval is a
quarter of a minute, you are secure.--_Brande's Lectures.--Lancet._
_New Crane._
A crane for raising weights, on an entirely new principle--that of the
application of the lever, assisted by wedges, instead of the usual
plan of wheel and pinion, for multiplying power--has recently been
constructed at the West India Docks. The power of two men, with the
patent crane, is stated to be capable of lifting from 2-1/2 to 3 times
the weight lifted through the same space in a given time, by the best
constructed cranes on the old principle of wheel machinery.
_Etching on Ivory_.
The usual mode of ornamenting ivory in black, is to engrave the pattern
or design, and to fill up the cavities thus produced with hard black
varnish. Mr. Cathery has much improved and simplified the process, by
covering the ivory with engraver's varnish, and drawing the design with
an etching needle; he then pours on a menstruum, composed of 120 grains
of fine silver, dissolved in an ounce measure of nitric acid, and
diluted with one quart of pure distilled water. After half an hour, more
or less, according to the required depth of tint, the liquor is to be
poured off, and the surface is to be washed with distilled water, and
dried with blotting paper. It is then to be exposed to the light for
an hour, after, which the varnish may be removed by oil of turpentine.
The design will now appear permanently impressed on the ivory, and of
a black or blackish brown colour, which will come to its full tint after
exposure for a day or two to the light. Varieties of colour may be given
by substituting the salt of gold, platina, copper, &c. for the solution
of silver.--_Trans. of the Society of Arts._
_Geology._
Among the fossil bones lately dug from under the lava of the mountain
of Boulade, in the neighbourhood of Issoire, in France, none have been
discovered belonging to the human body. The same is the case in the
other mountains of the vicinity. But, although there are no human bones,
in several places, and especially in the mountain of Boutaresa, (which
is not far from the mountain of Boulade,) pieces of wood have been
discovered, buried under the ancient lava, which observers worthy of
credit declare seem to have been fashioned by the hand of man, and to
have been cut with a hatchet, although rudely, and as might be expected
in the infancy of the arts. Did man exist then, at that remote period
when elephants, lions, and tapirs, lived in Europe, with rein-deer and
bears? This is an exceedingly difficult question, and one which hitherto
does not, by any means, appear to have been satisfactorily
resolved.--_New Monthly Magazine_.
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