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._