He states the average time of working by the gas light throughout the year at two hours per day, this will require a daily consumption of gas equal to 2500 cubic feet ; and allowing 313 working days, the yearly consumption will be 782,500 cubic feet.
The veracity of this author would alone be sufficient to give great weight to these facts ; but they have gained greater strength by their strict agreement with subsequent experience.
It is now found that the best form for the retorts is a cy linder, and that they should not much exceed 10 inches in diameter, nor be much more than six feet in length. if they were much wider, the heat would not penetrate the loose coal within sufficiently ; and as regards the length, the fire would not act to much advantage, nor the heat be uniform, if the length were much beyond the above state ment. This should be the limit for one furnace. If more gas is wanted, more furnaces must be made, as has been shewn in the Plate.
In an apparatus on the scale of these in London for light ing the streets, from 24 to 36 such retorts and furnaces would be required. These retorts contain, independent of the mouth-pieces, about 5655 cubic inches, and will hold conveniently 100 lb. of cannel coal. When the fire is ap plied to good advantage, each retort gives out all its gas in about four hours. There is no good policy in pushing the distillation very far, as the gas which comes the last is the least combustible. The retort should not be heated beyond a brightish red, as the gas is not only injured, but the re tort would soon be destroyed.
In order to condense as much as possible the most use ful facts, we have given a Table, founded upon the state ments already made. We are indebted to Mr Murdoch for the comparative light given by gas and candles, the gas and coak afforded from a given quantity of coal, and the consumption of common coal to produce the necessary heat for the distillation. The proportion of tar and ammoniacal liquor we have quoted from Mr Accum's work on gas lights.
The first column gives the number of retorts, each being 10 inches in diameter, and 6 feet long. Tile second column gives the capacity of the gazometer. The third column, the weight of coal used to furnish the gas. The fourth and fifth columns give the same by measure. The sixth, the coal used to distil off the gas. The seventh, the cubic feet of purified gas. The eighth, the weight of coak left in the retorts. The ninth, the weight of tar. The tenth, the am
moniacal liquor ; and the eleventh, the number of candles to produce the same light with the gas ; the candles being six to the pound each, when fairly burnt, consuming 175 grains of tallow in one hour.
This 'fable is formed by doubling the first numbers of each column for the second number, then adding the first number to the second for the third, the third to the first for the fourth, and so on, so that it may be easily extended to any greater number of retorts.
If it were required to find the number of candles of any other size, or a different number in the pound, multiply the number of candles in the Table by the given number in the pound, and divide the product by 6, the number in the pound of those in the Table ; the quotient will be the num ber required. For example, the number of candles equal to the gas from 800 lb. of coal, which is 2560 cubic feet, is 768, what will be the number of candles of 5 to the pound ? ; then, 3840÷6=640, the number of candles required.
From this Table it may be seen how much coal is equal to a given weight of tallow. If the number of candles in the last column be divided by 6, the quotient will give the weight of tallow in pounds to the coal in the third column. For instance, the candies equal to 800 lb. of coal are 768 ; the last divided by 6 gives 1281b. of tallow, equal to 800111. of coal, which is 1 lb. of tallow to 6.25 of coal.
Gas light may be compared with candles in another point of view. That is, what number of candles of a given size give a light equal to that produced by the gas when its rate of burning is given ; that is, when a given quantity in cubic feet is burned in a given time. We have seen, by the data already gi'Ven, that half a cubic foot of gas will be consum ed in the same time, giving the same light as 175 grains of tallow from a candle 6 to the pound. We therefore should say, that when a cubic foot of air is burnt in two hours, its light will be equal to that of a mould candle of 6 to the pound ; but if the same gas were burned in one hour, either from doubling its velocity or its aperture, then it would re quire two such candles to produce an equal light in all re spects. If the candles to give an equal light with the gas when burnt in one hour be divided by the time, it will give the candles for that time. To find the quantity of candles of any other size, use the rule above given.