July i6, 1891] 



NATURE 



261 



firedamp was required, the methane, prepared and purified by 

 ordinary chemical methods, was introduced into the chamber in 

 the requisite quantity by the top inlet. It displaced an equal 

 volume of air, which escaped through the lower outlet, the exit 

 end of which was sealed by being immersed just beneath a water 

 surface. A vigorous use of the mixer secured a uniform mixture 

 of gas and air throughout the interior of the chamber in the 

 course of a few seconds. The lamp was then introduced into 

 the chamber, and placed in position behind the glass window. 

 The simplicity of arrangement of the water-seal rendered the 

 necessary opening of the chamber very brief, and the introduc- 

 tion and removal of the lamp many times in succession was not 

 found to produce any appreciable effect upon the composition of 

 the atmosphere inside the chamber. The appearance and 

 dimensions of the " cap " over the flame were noted as soon 

 as the cap underwent no further change. A lamp was left 

 burning in the chamber for a considerable length of time, and 

 its indications underwent no change, owing to the large capacity 

 of the chamber and the very limited amount of air required to 

 support the combustion of the small flame always used in gas- 

 testing. The whole interior of the chamber and mixer were 

 painted dead-black, so as to render visible pale and small caps 

 against a black ground. 



The methane was introduced from an ordinary gas-holder. 

 A volume of water, equal to that of the methane to be displaced, 

 was poured into the top of the gas-holder. The gas-tap of the 

 holder was then momentarily opened, so as to produce equi- 

 librium of pressure between the methane and the atmosphere. 

 The gas-tap having then been placed in connection with the 

 upper inlet of the chamber, the water-tap was opened, and the 

 measured volume of water was allowed to flow down and drive the 

 methane into the chamber. As soon as bubbles of air ceased to 

 appear through the water at the outlet, the chamber was closed ; 

 the mixer was then vigorously worked for a few seconds, and 

 the mixture of gas and air was ready for the introduction of the 

 lamp. Before introducing the methane for a fresh mixture, the 

 atmosphere of the chamber was replaced by fresh air by re- 

 moving the water-tray from beneath the opening at the bottom 

 of the chamber, and blowing in a powerful stream of air from a 

 bellows to the top of the chamber. 



The chamber was supported on legs, which were arranged so 

 as to place it at a convenient height for observations through the 

 window, and also for the introduction and removal of the safety- 

 lamp. 



The observations were usually made in a darkened room, but 

 the flame-caps were easily seen in a lighted room, provided direct 

 light falling on the eye or chamber was avoided. 



The capacity of the chamber was 95,220 c.c. ; accordingly, 

 the following volumes of methane were introduced : for 5 per 

 cent, mixture 476 c.c, for I per cent. 952 c.c, for 2 per cent. 

 1904 c.c, for 3 per cent. 2856 c.c, for 4 per cent. 3808 c.c, 

 and for 5 per cent. 4760 c.c. It will be seen that a series of 

 tests, in which the above-mentioned percentage mixtures were 

 employed, involves an expenditure of only 15 litres of methane, a 

 quantity far smaller than that required by any other method of 

 testing as yet' described. 



Of many forms of safety-lamp tested in the above apparatus, 

 the one which most satisfactorily fulfilled the two purposes of 

 efficient illumination and delicacy in gas-testing was Ashworth's 

 improved lie pple white- Gray lamp. This lamp is of special 

 construction, burns benzoline from a sponge reservoir, and its 

 flame is surrounded with a glass cylinder, which is ground rough 

 at the hinder part ; this latter device prevents the numerous 

 reflected images of the flame, and the generally diffused reflec- 

 tions which are seen from a smooth glass surface, and which 

 render the observation of a small pale flame-cap very difficult, 

 if not impossible. 



The wick of this lamp, when at a normal height, furnishes a 

 flame of great illuminating power. When lowered by a fine 

 screw adjustment the flame becomes blue and non-luminous, 

 and does not interfere therefore with the easy observation of a 

 pale cap. The following heights of flame-cap were observed, 

 which fully bear out the unusual sensitiveness of this flame. 

 With 05 per cent, of methane 7 mm. ; with i per cent. 10 

 mm. ; with 2 per cent. 14 mm. ; with 3 per cent. 20 mm. ; with 

 4 per cent. 25 mm. ; and with 5 per cent. 30 mm. The cap, 

 which with the lower proportions was somewhat ill-defined, 

 became remarkably sharp and definite when 3 per cent, and up- 

 wards of methane was present. But even the lowest percentage 

 gave a cap easily seen by an inexperienced observer. 



NO. I I 33, VOL. 44] 



It appears from the above record of tests that the problem of 

 producing a lamp which shall serve both for efficient illuminating 

 and for delicate gas-testing purposes has been solved. The solu- 

 tion is in some measure due to the substitution of benzoline for 

 oil, since the flame of an oil-lamp cannot be altogether deprived 

 of its yellow luminous tip, without serious risk of total extinc- 

 tion ; and this faint luminosity is sufficient to prevent pale caps 

 from being seen. 



From further experiments made in the above testing-chamber 

 with flames produced by alcohol and by hydrogen, it was found 

 to be true in practice, as might be inferred from theory, that, if 

 the flame was pale and practically non-luminous, the size and 

 definition of the flame-cap was augmented by increasing either 

 the size or the temperature of the flame. It is quite possible by 

 attending to these conditions to obtain a flame which, although 

 it is very sensitive for low percentages of gas, becoiftes unsuit- 

 able for the measurement of any proportion of gas exceeding 3 

 per cent. This must, for the general purposes of the miner, be 

 looked upon as a defect ; but it is not a fault of the lamp already 

 referred to. It is of interest to note that with the Pieler spirit- 

 lamp a flame-cap an inch in height was seen in air containing 

 only o'5 per cent, of methane. 



Physical Society, June 26.— Prof. W. E. Ayrton, F.R.S., 

 President, in the chair. — The following communications were 

 made : — The construction of non-inductive resistances, by Prof. 

 W. E. Ayrton, F.R.S., and Mr. T. Mather. In making some 

 transformer tests about three years ago, the authors had occasion 

 to consider the construction of electric conductors the imped- 

 ances of which should be practically equal to their resistances. 

 This condition could only be fulfilled by making the inductance 

 small in comparison with the resistance ; and, as the former 

 does not depend on the material employed (excepting iron) it 

 was important to use substances of high specific resistance. 

 Carbon or platinoid being available, the latter was chosen on 

 account of its low temperature coefficient. One form of resist- 

 ance exhibited consisted of strips of thin sheet platinoid; 

 about 6 metres long and 4 centimetres wide. Each was bent 

 at the middle and doubled back on itself, thin silk being 

 placed between the contiguous parts and narrow ribbon used to 

 bind the parts together. Twelve such strips arranged in series 

 had a resistance of 2*95 ohms, and would carry a current of 15, 

 amperes without changing its resistance more than ^V pc" cent. 

 This strip-resistance was made by Messrs. C. G. Lamb and E. 

 W. Smith, who at that time (1888) were students in the Central 

 Institution, and to whom the author's best thanks are due for 

 the praiseworthy manner in which they surmounted the diffi- 

 culties which presented themselves. Another form of resistance 

 designed for portability consisted of bare- wire spirals, each 

 length having a left-handed spiral placed within a right-handed 

 one of slightly larger diameter, and the two being connected in 

 parallel. This device was found to reduce the inductance to 

 tV or ifV of that of a single spiral according as the diameters of 

 the spfrals approach towards equality. Where the spirals are 

 made of platinoid wire, the ratio of inductance to resistance is 

 very small, averaging about TnyoVso- — On the influence of .surface 

 loading on the flexure of beams, by Prof. C. A. Cams- Wilson. 

 Referring to the practical treatment of problems on beam flexure 

 as based on Bernoulli's hypothesis that the bending moment 

 is proportional to the curvature, the author pointed out that this 

 assumes that the cross-sections remain plane after flexure, and 

 neglects the surface loading effect. The present paper describes 

 experiments made to determine the actual state of strain in a 

 beam doubly supported, and carrying a single load at the centre, 

 the effect of surface loading being taken into account. The 

 method of investigation assumes that (i) the true state of strain 

 at the centre of a beam may be found by superposing on the 

 state of strain due to bending only, that due to surface loading 

 without bending ; (2) the state of strain due to surface loading 

 only, may be found with close approximation to truth by resting 

 the beam on a flat plane instead of on two supports ; 

 {3) the strain due to bending alone, may be obtained from the 

 Bernoulli-Saint-Venant results. Before proceeding to describe 

 the experiments, a short account of the mathematical work pre- 

 viously done on the subject was given. The nearest approach 

 to the particular case here dealt with had been worked out by 

 Prof. Boussinesq, who had shown that for an infinite elastic 

 solid bounded on one side by a plane surface and loaded along a 

 line on that surface, the stress (/) on an element on the normal 

 through the middle point of the line varies inversely as its dis-. 



