April 3, 1893] 



NATURE 



517 



Prof. Lewes's paper on the ignition of coal cargoes was 

 quite a new departure in the practice of the Institution. 

 When the members assembled they found an array of 

 bottles, flasks, and chemical apparatus, that was not a little 

 puzzling to those not in the secret, and must have reminded 

 many of the dear old Polytechnx days and Prof. Pepper. 

 However, the lecture, and the experiments by which it 

 was illustrated, were of a thoroughly sound and prac- 

 tical nature. The question of spontaneous ignition of coal 

 cargoes is one for the ship-owner rather than the ship- 

 builder ; excepting that ship-builders have to replace the 

 vessels which are destroyed by reason of such spon- 

 taneous ignition. The lecturer illustrated the influence 

 of carbon in producing heating by the power it possesses 

 ■of attracting and condensing gases upon its surface. The 

 action of the bituminous constituents of the coal in spon- 

 taneous ignition was next dealt with, and the author then 

 proceeded to point out the important part the action of 

 iron disulphide, pyrites, or coal-brasses played in pro- 

 moting spontaneous ignition. The remedy Prof. Lewes 

 advises for the evils of spontaneous ignition are : 

 firstly, non-ventilation of holds, so that oxygen may 

 not be admitted to carry on the chemical processes by 

 which heat is generated ; secondly, by placing thermo- 

 meters, suitably protected, in the mass of coal, so that, 

 by electrical communication, warning may be given when 

 the temperature rises to a dangerous point ; and, thirdly, 

 by placing flasks of liquid carbonic anhydride in the 

 coals, the flasks to be sealed by an alloy with a low 

 melting-point. This would be fused when the dangerous 

 temperature was reached, and the carbonic acid, in 

 expanding to its gaseous state, would cool the mass of 

 coal to a safe temperature. 



At the last sitting of the meeting, Mr. Corbett's paper 

 on lifeboat models raised a lively controversy. The Royal 

 National Lifeboat Institution had brought Mr. G. L. 

 Watson all the way from Glasgow to meet the bold 

 innovator who proposed to abolish their cherished self- 

 righting boats. Of course, who is right remained an 

 -open question, as it always does when the properties of 

 lifeboats are concerned. 



Mr. Howden's paper on the screw propeller was of 

 great length, containing no less than twenty-four pages 

 without the appendix. Mr. Howden, like many other 

 people, has a theory of his own on the screw propeller, 

 which is opposed to that of all other authorities on the 

 subject ; for he believes that Rankine, Froude, Cotterill, 

 and others, have based their conclusions on erroneous 

 premises. It will be evident that we cannot enter into 

 this vast subject at the end of a notice such as this, but 

 ^ve may briefly record our opinion that the older authori- 

 ties were right. 



On the whole, the meeting passed off very well. The 

 attendance was good, and Mr. Holmes, the secretary, 

 had made his arrangements so that the business pro- 

 ceeded without a hitch, as, indeed, is invariably the case 

 at this well-managed institution. 



BOURDON'S PRESSURE GAUGE. 



]\/r R. WORTHINGTON'S letter to Nature, January 

 ■'■*-*• 30 (p. 296), on the theory of this instrument, has 

 excited some criticism and disagreement of opinion ; so 

 it is proposed to examine here how far it is possible to 

 •construct a theory which shall be quantitative, in addition 

 to giving a general explanation of the action. 



The instrument is in very extensive use, hardly a 

 steam-boiler being in existence which is not provided 

 with one ; and the simplicity and strength of the con- 

 struction are such that it does not easily get out of repair, 

 while it can be made to register either the highest pressure 

 of the hydraulic press, or to record in the form of a baro- 

 meter the minute fluctuations of atmospheric pressure. 



The principle of the instrument was discovered by 

 accident, and the account of this had best be given in 

 the inventor's own words, taken from the paper read by 

 him before the Institution of Civil Engineers, printed in 

 the Proce6dings I.C.E., vol. xi., p. 14, 1851 : — 



" The author had occasion to construct a worm-pipe for 

 a still, by bending a cylindrical tube into a spiral or 

 helical form. The workman performed the operation 

 awkwardly, and partially flattened a considerable portion 

 of the tube. In order to restore its form, one end was 

 closed and the other was connected with a force- pump, by 

 which water was forced into the tube ; as the flattened 

 portion of the tube resumed its cylindrical form, it was 

 observed that the spiral uncoiled itself to a certain extent, 

 and it was immediately perceived that this action might 

 be applied to the construction of a pressure gauge." 



To construct, then, a Bourdon gauge to register high 

 pressures {vu/e figure, representing a gauge fitted to an 

 indicator, not shown) a steel tube bored out of the solid 

 bar to the requisite thickness for strength is taken, and 

 purposely flattened, and then bent round into the arc of 

 a circle so that the longer axis of a cross-section stands 

 at right angles to the plane of the circle : one end of the 



tube is screwed to a pipe which communicates with the 

 liquid whose pressure is to be measured, while the other 

 end is closed and joined by levers and racks to a shaft 

 and a pointer, which traverses a dial on a box in which 

 the curved tube is enclosed. 



As the pressure in the tube is increased, the circular 

 axis uncoils into a larger circle of smaller curvature, and 

 the corresponding indications of the pointer on the dial 

 are marked ; and thus the instrument is graduated 

 empirically by reference to some standard pressure gauge. 

 As the pressure is again diminished, the elasticity of the 

 tube brings it back to its original form, and the pointer 

 retraverses the dial. 



Lord Rayleigh gives an -elementary explanation of the 

 action of Bourdon's gauge in the Proc. Royal Society, 

 No. 274, December 13, 1888; treating the movement of 

 the walls of the tube as one of pure bending, he says : — 



" In this instrument there is a tube whose axis lies along 

 an arc of a circle and whose section is elliptical, the 

 longer axis of the ellipse being perpendicular to the 

 general plane of the tube. If we now consider the 

 curvature at points which lie upon the axial section, 

 we learn from Gauss's theorem (that in the bend- 

 ing without stretching of an inextensible surface, the 



