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