564 GAS, COAL 



flamo, and at the opposite end tho standard candle. A stand which slides easily 

 along tho rod supports a small circular paper screen, at tho same height as tho two 

 flames, and at right angles to the rod. This screen consists of colourless, moderately 

 thin writing paper, saturated with a solution of spermaceti in spirit of turpentine, 

 except a spot in the centre, about the size of a shilling, which is to bo left untouched 

 by the solution. The spirit of turpentine soon evaporates, and the paper is now ready 

 for use. Being more transparent in tho portion which has been saturated with the 

 spermaceti solution, it becomes a delicate test of equality of light when placed between 

 two luminous bodies ; for if the light of one of the bodies impinge with greater in- 

 tensity upon one side of tho screen than the other light docs upon tho opposite side, 

 tho difference of tho transparency of tho two portions of the screen will become dis- 

 tinctly visible ; tho spot in the centre appearing comparatively opaquo on tho less 

 illuminated side. When tho screen is brought into such a position between tho two 

 sources of light as to render the central point nearly or quite invisible on both sides, 

 tho illuminating effect of both lights at that point may be regarded as equal ; and 

 all that now remains to be done is to measure the respective distances of the candlo 

 and gas from tho screen, and divide the square of the distance of tho gas by tho 

 square of that of the candle ; tho quotient expresses the illuminating power of tho 

 gas in candles. One of tho most convenient forms of this instrument has been con- 

 trived by Mr. Wright, and may be had at 65 Millbank Street, Westminster. It 

 consists of the following parts : 



1. A wooden rod, exactly 100 inches long (fig. 1019) from the centres of sockets at 

 its ends, A B. 



2. An upright pillar, c. 



3. A candle holder, D. 



4. A mahogany slide, E, having a metal socket, F, on its top, to hold the circular 

 frame o, and a small pointer in its front. 



5. A circular metal frame, o, made to hold a prepared paper. 



6. A blackened conical screen, H, diminishing in size from its centre, where it 

 opens with a hinge towards its ends, with two holes in front. 



The long rod is graduated in accordance with the laws of distribution of light, from 

 its centre each way into squares of distances in divisions numbered respectively 1, 2, 

 3, &c., to 36 ; to measure smaller differences than those amounting to 1 candle in 

 value, each major division to 9 is subdivided into 10 parts, each, of course, repre- 

 senting ^th of an increment. From thenco to 20, the subdivisions indicate . Beyond 

 that point no subdivisions are made, because the major divisions become so small 

 that, practically, such divisions would bo useless. 



The manner of fitting the apparatus together will be understood by reference to the 

 annexed sketch. 



1019 



The pillar, c, is screwed to one end of tho shelf, and an experimental meter, t, 

 placed at tho other. This latter instrument is for measuring tho quantity of gas 

 passing to the burner, and indicating tho rate of consumption by observations of ono 

 minute, which is accomplished by tho construction of its index dial. 



This dial has two circles upon its face, with a pointer to each ; the outer circle 

 divided into four, and the inner into six parts; and each of these again divided 

 into tenths. Every major division of tho outer circle is a cubic foot ; and every 

 major division of the inner circle is -^th of a cubic foot ; so that the major divisi >ns 

 on the inner circle each bc.'ir the same proportion to a cubic foot that a minute 

 does to an hour. If, therefore, the number of these divisions and tenths of divi- 

 sions, which tho hand passes over in a minulr. i.s observed, it will evidently only be 

 ry to read them off as feet and tenths of a foot to obtain tho hourly rate of con- 

 sumption. 



Thus, suppose the pointer passes from tho upper figure 6 to the fifth minor division 

 beyond tho figure 4, it would read off as 4-^ths and c^ths of a cubic foot in Jjth of an 



