536 EEPORT— 1886. 



mental standard of liglit, and showed two of the standards to which reference was 

 made in the report of a committee appointed by the Board of Trade in 1881. 



These were the Harcourt air-gas lamp and the Methven screen. 



The improvements made in the former since the last meeting of the British 

 Association consisted in an adjustable black background and screen for protecting 

 the eye from the light of all but the upper point of the flame when regulating the 

 height, and thereby enabling the exact height to be determined more accurately ; 

 also in a rack and pinion movement with a scale engraved in millimetres for 

 setting the height of the platinum wire ; and besides these an entirely new method 

 of preserving an accurately even rate of drop of pentane for feeding the lamp in 

 the portable form exhibited. 



This device consists in producing a perfectly constant head by providing an 

 overflow outlet from which the excess of pentane drops into a small bottle, which 

 can be removed when necessary, and emptied into the main reservoir, which is on 

 the top of the lamp, the bottle forming a stopper to the reservoir to prevent 

 evaporation. 



The rate of drop into the lamp is regulated with great delicacy by letting the 

 pentane flow down a fine glass tube, in which there is a constricted passage, which 

 can be more or less closed by a fine platinum wire which can be screwed in or out 

 of it by means of a thread working in a cap at the top of the tube. This method 

 is due to Mr. W. F. Donkin. 



In other respects the lamp remains exactly as described by Mr. Vernon Harcourt 

 at previous meetings of the Association. 



The Methven screen was one of the form as now constructed, but differing from 

 that upon which the committee on photometric standards reported in 1881. The 

 author showed by a diagram the errors introduced by the alteration of form, 

 amounting to fully 16 per cent, below its normal value. 



He gave the result of observations made by Mr. W. F. Donkin and himself, 

 and showed that the errors may be determined theoretically, and were practically 

 coincident with the result of observations. 



The value of the Methven varied from 1687 at 65" to 2-149 at 11". 



Observations also showed that the increased thickness of flame subtended by 

 the disc as it approached the slot caused an increase in the value. 



Allusion was made to experiments for determining the absorption of light 

 by cylindrical glass chimneys, and it was stated that whereas frosted glass absorbs 

 30-40 per cent, of the light a glass cylinder frosted on the outside only absorbs 

 from 7-15 per cent. 



17. Fuel Galorimttry} By B. H. Thwaite, F.C.8. 



Although instruments for the precise estimation of most of the agents of our 

 industries have long ago been introduced, the heating value of coal — the great 

 natural source of power — is rarely tested calorimetrically, even by the largest 

 consumers. 



At present the different qualities of coals are known as bests, seconds, &c. 

 whereas a calorific estimation might show that the seconds, or even inferior 

 quaUties, possessed a higher calorific efficiency than the Jirsts. 



By the utilisation of fuel calorimetry a user of coal would be able to ascertain 

 exactly the financial value of diflferent fuels, and to compare the heat energy pos- 

 sessed by the fuel with that economically evolved. 



Fuel calorimetry would prove a strong inducement to the adoption of more 

 perfect combustion arrangements, and thus aid the laudable objects of the Smoke 

 Abatement Society. 



The disadvantage of Dulong's calorimeter, in which the fuel is consumed in a 

 current of oxygen, is that the combustion occupies a considerable time, and conse- 

 quently requires correction for re-cooling ; moreover, unless the oxj'gen is applied 

 by compression, the instability of carbon dioxide in the presence of carbon prevents 



' See Engineeririg, November 12, 1886, p. 507. 



