188 REPORT— 1864. 



age of 0'5 of her length, and that the racking action cannot exceed '16 of her 

 weight. Applying those results to two remarlvabh^ good examples of ships of 2080 

 tons displacement, one of iron and tlie other of wood, described by Mr. John Vernon 

 in a paper read at the Institution of Mechanical Engineers in 1863, he finds the fol- 

 lowing values of the greatest stress of different kinds exerted on the materials of 

 the ship : — 



In the ii'on shi^), tension 3-92 tons per square inch. 



„ „ thrust 2-35 „ „ 



,, „ racking stress 1-4-4 „ „ 



It follows that, in the iron ship, the factor of safety against bending is between five 

 and six, agreeing exactly with the best practice of engineers, and that there is a 

 great surplus of strength against racking : — 



In the wooden ship, tension 0'371 tons per square inch. 



„ „ thrust 0-239 „ „ 



Here the factors of safety are between ten and fifteen, which also agrees with good 

 practice in carpentry. As for the racking action, the iron diagonal braces required 

 by Lloyd's Rules would be sufficient to bear one-fifth of it only, leaving the rest 

 to be borne by the friction and adhesion at the seams of the planking. 



On Units of Measure. 

 Bij W. J. MAcairoEN Eankine, C.E., LL.D., F.R.S. 

 Professor Rankine, one of tlie Committee, dissented from that part of the Report 

 which recommended the abandonment of the British units of measure, and read a 

 paper, in which he arrived at the conclusion that while the advantages of decimal 

 mid'tiplication and division as applied to units of measure are incontestable, the 

 question between different units, sucli as the metre and the inch, is one of conve- 

 nience, in which the interests of science and of trade cannot be separated ; and 

 that inasmuch as the British inch and multiples of the inch are already establislied 

 by law and custom and used for practical purposes, in regions inhabited by one- 

 fourth of mankind, their use oua'ht not to be abandoned in scientific writings. 



On Submarine Telegraphy. Bij Captain Selwvn, R.N. 



The paper commences by pointing out possible or probable causes of the failure 

 and loss which have hitherto been lamentably prominent features in these great 

 enterprises. The author considers tliat there is not the slightest reason to doubt 

 that gutta-percha, properly laid at the bottom of the sea, in whatever depth, is a 

 perfect and reliable insulator of electricity, reasoning from the fact that many of 

 the shallow-water cables have been down from eight to twelve years. The prior 

 failures are ascribed to faulty mechanical construction of the cable and faulty 

 mechanical arrangements for its deposition on the bed of the ocean. The life of 

 the cable, that which must not be injured in any case, is the copper wire that con- 

 veys the electricity. A stretching of this, even to the extent of one in a hundred 

 (which, be it recollected, means perhaps one mile in a hundred), cannot for an 

 instant be admitted. Yet this wire is placed in the centre of a comparative^ soft 

 and absolutely weak core, and surrounded with spirals of iron or steel by way of 

 giving strength. The axiom of mechanics which is here transgressed is this : — 

 In any structure composed of spirals in combination with straight lines, any strain 

 must first be borne by the straight lines. It was stated that the best insulator is 

 the compound of Mr. John Macintosh, one-half cheaper than either gutta-percha 

 or india-rubber, and much superior to either gum, both in goodness of insulation 

 and loAvness of inductive capacity. AVitli regard to the route which it is advisable 

 to pursue : recent discoveries of shoal water, 80 furlongs halfway, lat. 43° 30' N., 

 long. 38° 50' W., in the direct great-circle tract between this country and the 

 island of Bermuda make it certain that means may be found of dividing any future 

 cable into comparatively short sections. Captain Selwyn is of opinion that a 

 species of vulcanized rubber coating will be found the best and cheapest protecting 

 material. Captain Selwyn's plan for pajdng out the cable consists m the employ- 

 ment of one or more cylindrical drums, built of sheet iron or wood, as strongly 



