MECHANICS AND USEFUL ARTS. 101 



NEW METHOD OF TESTING SUBMARINE TELEGRAPH CABLES. 



A plan devised by Messrs. Reed, of England, engineers, for effectually 

 testing submarine telegraph cables previous to their deposition, so that any 

 defect may be made evident, consists in first exhausting the air from a ves- 

 sel in which the cable to be tested is placed, and then forcing in water, until 

 a pressure of about two hundred pounds per square inch is attained. To 

 perform this operation, they employ a vessel which is so constructed as to be 

 possessed of sufficient strength, so as to resist the pressure of the atmos- 

 phere when exhausted, and also, at the same time, the hydrostatic pressure 

 to which the cable is to be subjected. The vessel in which the operation 

 takes place is provided with a cover, so as to admit of a coil of insulated 

 wire being introduced and inclosed therein. One end of the covered wire is 

 conducted from the interior, through a shifting box, to the outside of the 

 vessel ; the other end of the wire is coated over as well, and insulated. All 

 being thus arranged, a vacuum is formed, by means of air-pumps, in the 

 vessel which contains the cable. The stop-cock of the air-pumps is now 

 shut off, and the passage for the water is opened, so as to admit of the water 

 entering into the vessel to fill it ; or, if desired, a quantity of water may be 

 allowed to enter into the vessel, so as to fill it, or nearly so, before the pneu- 

 matic apparatus is put into action. One end of the wire of a galvanometer is 

 connected with the outer end of the wire which has been brought through 

 the vessel, as described above. Pressure is next exerted by pumping water 

 into the vessel, and then, on connecting the two poles of the battery with 

 the galvanometer and the water in the vessel respectively, if the insulation 

 be perfect, no action takes place in regard to the needle of the galvanometer, 

 as no complete electric current has been formed; but, on the contrary, 

 should there exist any defect whatsoever in the coating of the wire, however 

 small it may be, the needle of the galvanometer will at once, by its deflec- 

 tion, indicate the same to the operator, which shows that a circuit has been 

 formed, owing to some of the water in the vessel getting into contact with 

 some part of the wire which is being tested. 



OILED PAPER AS A SUBSTITUTE FOR OILED SILK AND GUTTA- 

 PERCHA IN SURGICAL DRESSINGS. 



This material was introduced by Dr. James McGhie, of the Glasgow Royal 

 Infirmary, and has been used with success in hospital practice. 



The following is the mode of preparing it : 



Having secured a paper of good texture, the next desideratum is the fluid 

 or varnish by which it is to be coated and waterproofed. This is made by 

 rebelling boiled linseed oil with litharge, acetate of lead, sulphate of zinc, 

 and burnt umber, an ounce or two of each to a gallon of oil. No artificial 

 heat is employed in drying. A square board is now procured, several inches 

 broader than the size of the sheet to be prepared. Upon this the sheet is 

 spread, and well covered, by means of a broad brush, with the mixture. 

 The first sheet should be brushed on both sides. On this a second sheet is 

 placed, slightly projecting over the first, at one end, in order to facilitate the 

 lifting of the sheets when they are to be hung up to dry. This is also to be 

 coated with the mixture. This process is to be repeated till a mass of 

 sheets, from twenty to fifty in number, is prepared. The board is then to 

 be carried to some unoccupied apartment, across which cords have been 



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