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 reason further, only his system of distribution is considered, though it may be a question whether it is the one which will prove most satisfactory 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 batteries 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, covering 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 exhibited in the following table, in percentages of the total make for the twenty-four hours:
| 7-8 | a. m. | 1½ | per cent. | 7-8 | p. m. | 12 | per cent. | ||
| 8-9 | " | ½ | " | 8-9 | " | 12 | " | ||
| 9-10 | " | ½ | " | 9-10 | " | 10 | " | ||
| 10-11 | " | ½ | " | 10-11 | " | 6 | " | ||
| 11-12 | " | ½ | " | 11-12 | " | 5 | " | ||
| 12-1 | p. m. | ½ | " | 12-1 | a. m. | 17/10 | " | ||
| 1-2 | " | ½ | " | 1-2 | " | 17/10 | " | ||
| 2-3 | " | ½ | " | 2-3 | " | 17/10 | " | ||
| 3-4 | " | ½ | " | 3-4 | " | 17/10 | " | ||
| 4-5 | " | 7½ | " | 4-5 | " | 17/10 | " | ||
| 5-6 | " | 16 to 20 | " | 5-6 | " | 16/10 | " | ||
| 6-7 | " | 14 | " | 6-7 | " | 17/10 | " |
