C. GENERAL PHYSICS. 



191 



light of the Gramme machine in illumination, undertaken at 

 the request of the Compagnie du ISTord, their stations being 

 used for the purpose. The machines selected for the trials 

 were those rated at 50, 100, and 150 Carcel burners respect- 

 ively, equivalent to 350, 700, and 1050 candles* The experi- 

 ments were made in the baggage-room and in the station it- 

 self; the former having an area of about 16,000 square feet 

 and a volume of about 070,000 cubic feet, the latter an area 

 of nearly 120,000, and a volume of 10,500,000 feet. The 

 force used was derived from both steam and gas motors, the 

 power being measured with a Prony brake. The Serrin 

 lamp was employed. The results obtained are tabulated as 

 follows : 



Power required to in- 

 sure a steady light. . . 



Revolutious per miuute 



carbons, 6 milli- 

 meters sq 



carbons, 9 milli- 

 meters sq 



Consumption r carbon, T mm... \ f^^t' P^|^' 

 of carbon and] nej;. po e, 



^^^t ^<^^'^b"'9mm...{;;2-j;*^;; 



Distance at which reading was easy 



Machine 1. 



1650 

 2.2 h. p. 



11 

 (I 



(4 



25 meters. 



Machine 3. 



800 

 2.4 h. p. 



2.6 " 



0.090 meter. 

 0.045 ' 

 0.060 ' 

 0.030 ' 

 40 to 45 mtrs. 



I 



Machine 3. 



800 

 2.5 h. p. 



2.7 " 



0.13 meter. 



0.09 " 



45 to 58 mtrs. 



As M. Tresca showed, the force necessary to produce a 

 light, say of 700 candles, increases rapidly as the total 

 light diminishes. The light being 12,950, 2100, 1050, 700, 

 and 350 candles, the force required is 0.415, 0.920, 1.7, 2.4, 

 and 4.4 horse-powers respectively per 700 candle-lights. 

 Moreover, it will be observed that a little more force is 

 needed to sret the lig-ht from carbons nine millimeters 

 square than from those which are seven. Calculating the 

 expense of this light as compared with gas, Sartiaux finds 

 that a gas-light of 700 candles requires a consumption of 

 15.75 cubic meters of gas per hour, which, at the price of 

 0.3 franc per cubic meter, would cost 5.7 francs ($1.14). 

 The electric light of the same power requires 2.7 horse- 

 power, which, at 0.09 franc per hour, is 0.24 franc. Add 

 to this 0.09 for the carbons, 0.45 for engineer, 0.20 for inter- 

 est, etc., the total expense is 0.98 franc (nearly 20 cents) ; 

 being between a fifth and a sixth of the expense of gas. 

 Moreover, the greater extent of surface illuminated increases 

 this difference exceedino-lv ; since to 2:ive the same illuraina- 

 tion with gas would certainly require at least twenty-five 



