57* 



THE POPULAR SCIENCE MONTHLY. 



larger. The figures for the electric plant are based upon the work of 

 Mr. Edison, as he is the only one who has so far made any attempt 

 to put in an electric plant upon an industrial scale. And, for that rea- 

 son further, only his system of distribution is considered, though it 

 may be a question whether it is the one which will prove most satis- 

 factory in practice. An objection to it of considerable force in the 

 opinion of some, is, the difficulty of handling engines and boilers with 

 sufficient rapidity to meet great and sudden variations of demand, such 

 as not unfrequently occur during the seasons of the year in which the 

 weather is changeable. The variation that experience has shown takes 

 place at different periods of the day can be met readily enough. On 

 this account, and on account of the greater freedom secured in the 

 matter of working various pieces of apparatus without interference, it 

 would seem that the system of distribution which includes a storage- 

 battery would be preferable, and may, perhaps, become the final form, 

 adopted in electric installations. It can not well enter into the present 

 calculation, however, as there are no data with reference to the first 

 cost and depreciation available, and because the present secondary bat- 

 teries do not seem to have yet reached a satisfactory commercial form. 



The cost of such a plant for coal-gas will vary in this country from 

 $2,500 to $4,000 for each million feet of the yearly make, but $3,000 

 may be taken as a fair average. Owing to the great variability in the 

 demand for light at different seasons of the year, a gas-works of this 

 size will be called upon to furnish but 200,000,000 instead of 365,000,000. 

 feet a year. The plant will therefore cost $600,000. Of this, $250,000 

 may be taken as the cost of the mains, which, in average conditions, 

 will have, for a works of this size, a total length of fifty miles, cover- 

 ing a district of about three square miles. To compare an electric 

 with a gas plant, it is necessary to know the number of five-foot 

 burners that will be maintained at the time of greatest consumption, 

 as on this depend both the amount of horse-power required and the 

 size of the mains to transmit the current. 



The variation in the demand for light from hour to hour, as it 

 would occur in average conditions on a bright December day, is ex- 

 hibited in the following table, in percentages of the total make for the 

 twenty-four hours : 



1-8a.m li 



8- 9 " \ 



9-10 " \ 



10-11 " 



11-12 " \ 



12- 1 p. m \ 



7- 8 p. m 12 per cent. 



12 

 10 



6 



5 



lft 



lft 



lft 

 lft 

 lft 

 lft 

 lft 



u 



U 



t< 



CI 



