40 Mr. Joseph W, Swan [March 10, 



made is too favourable, I think but little, if any, greater allowance 

 need be made for the charge in connection with the renewal of 

 lamps in electric lighting than ought to be made for the corresponding 

 charges for the renewal of gas-burners, globes, chimneys, &c. But it 

 will be seen that even if the cost for renewal of lamps should prove 

 to be considerably greater than the corresponding expense in the case 

 of gas, there is a wide margin to meet them before we have reached 

 the limit of the cost of gas lighting. 



I think too it must be fairly taken into account and placed to the 

 credit of electric lighting, that by this mode of lighting there is 

 entire avoidance of the damage to furnishings and decorations of 

 houses, to books, pictures, and to goods in shops, which is caused 

 through lighting by gas, and which entails a large expenditure for 

 repair, and a large amount of loss which is irreparable. 



I have based these computations of cost of electric light on the 

 supposition that the light product of 1 horse-power is 150 candles. 

 But if durability of the lamps had not to be considered, and it were 

 an abstract question how much light can be obtained through the 

 medium of an incandescent filament of carbon, then one might, with- 

 out deviating from ascertained fact, have spoken of a very much larger 

 amount of light as obtainable by this expenditure of motive power. 

 I might have assumed double or even more than double the light for 

 this expenditure. Certainly double and treble the result I have 

 supposed can actually be obtained. The figures I have taken aie 

 those which consist with long life to the lamps. If we take more 

 light for a given expenditure of power, we shall have to renew the 

 lamf>s oftener, and so what we gain in one way we lose in another. 

 But it is extremely probable that a higher degree of incandescence 

 than that on which I have based my calculations of cost, may prove 

 to be compatible with durability of the lamps. In that case, the 

 economy of electric lighting will bo greater than I have stated. 



In comparing the cost of producing light by gas and by electricity, 

 I have only dealt with the radical item of coal in both cases. Gas 

 lighting is entirely dependent upon coal — electric lighting is not, 

 but in all probability coal will be the chief source of energy in the 

 case of electric lighting also. When, however, water power is avail- 

 able, electric lighting is in a position of still greater advantage, and, 

 in point of cost, altogether beyond comparison with other means of 

 producing light. 



To complete the comparison between the cost of electric light and 

 gas light, we must consider not only the amount of coal required to 

 yield a certain product of light in the one case and in the other, but 

 also the cost of converting the coal into electric current and into gas ; 

 that is to say, the cost of manufacture of electricity and the cost 

 OF MANUFACTURE of gas. I caunot speak with the same exactness of 

 detail on this point, as I did on the comparative cost of the raw 

 material. But if you consider the nature of the process of gas manu- 

 facture, and that it is a process, in so far as the lifting of coal by 



