The i'fForts which havo hoen made, from Urne to time, to engrave on metala by means of electiicity, geems at last to have resulted in tho attainment of somothiug practical. An ingenious mechanic has produced an invontion by which a metal plato, upon whioh a detign isdrawn with a chemical ink of sorae kind, is slowly rotated with the fuco vrrtical, and several other similar platos, gradcd in size, aro also slowly rotated by appropriate mochanisui. The object ot' the invjntion is to engravu on tho smaller platos tho design traced upon the largest, on different soales of magnitude, which is accomplished by applyiag a eutting point to tho face of oach plato, and which is pressed against it by meaus of an olectrlc current whenevor a blunt point, applied to the largo plato, encountors tho iiik in which the design is tracod- the cutting points boing at other tiiims withdrawn. Tho poiut presented to tho first plato is inerely a "feeler," which deterniiues, by eloctrical agency, whother the ink is beneath it or not. If it ia, tho points are prossed iuto thu surface of tho other platos ; if not, they aro witb.dr.iwn and prevonted froin cutting. The feeier and tho burins all follow a spiral track. WATER AND (JAS PEB2CBATI0ÏT. Whon iron pipos, somo containing gas and tho othors water, have been laid in contiguity, the gas has in somo instancos beun found to escape t'roui one pipo and parineato through the other, until it mixed with the trator in it - a phcnomenon which, though extraordiuary, takes placo siniply in aocordance with the cheinical tlioury of the subtlo diffusion of gasea. Tho gas, having onco escapcd from ita own confinoment, imprégnate theground lyirg about tho pipoa, and thus tho pipe containing the wator is surroundud by gas. It thoreforo occupies tho position of a porous medium betweea two fluida of different donsitiCs, and the rainedy is obviously to diiuiuish tho porosity of the pipo ; this may bo effected cither by thc omi)lüyint'nt of anothor material, or by coating tho (ron externally or internally wltli soine deseription of protoctive puint. It' this uieasuro were uffjctually, carried out, and tho pipos protoctod externally, a doublo eud would be answered- the permeation would bo anested, and the corrosión and gradual dostruction of tlie pipes matorially rotavdoíl, if not altogether provcnted. Tho plan of oovoring tho outsido of gas-pipes with tar has beon rocommended iov this purpose ; but, though a coating of tar may possess somo advantage in sueh a cnsts, it ís by no moans the best provoittiva Ihat might bo used. A protecting and preserving paint or coiupositioa oapablo oí' efifectually answeriug this and similar punosos, is a desiderátum. CAST IROS RA.ILUOADS. The adoption of cast iron for street railroads and tramways ia meeting with much favor in Beotlami. At meeting of the trustees of the Olyde Navigation Company of Glasgow, the engineer reportod tïiat a cast iron tramway which had been laid down on tho South Quay for trial had stood in a very satisfactory mannor tho most severo tests for more than four months. During th:s period the passing of railway and cart traffic bad been almost continuous, but the tramway showed no signs either of disnlaoement in line or lovel, or of any -wcar or need of repair in my way, being to all intonts and puxposes as perfect as when flret luid down. Utidortho eircuiHstances of the severe tests to which the tranrway had been submittcd, tlie results were considered very satisfactory, and the further use of this style of cast iron tramway was ï-coonimcndod for quays and yards.