completely discharged of the air, and filled with gas, by placing one of its ends upon a pipe or cock, of the same size, and passing a current of gas through it in the following manner. A projection is placed on each side of the flask, by which it is intended to be held in the hand, so that the temperature of the gas or air within may not be increased. Previously to the flask being filled with the gas intended to be weighed, let it be freed from foul air, or gas, by taking off the caps from each end, and passing a current of fresh air through it, until the air within is the same as without; this may be easily effected by moving it quickly and repeatedly to and fro, endways, in the air; the caps may then be put on, and the flask accurately balanced; this being done, take off the caps, and pass a current of gas through it during about ten seconds of time; then put on the cap at the upper end, and, at the instant of disengaging the bottom end of the flask from the pipe or cock, put on its cap, and immediately weigh it again, when the diminution, or counter-weight, required for restoring the balance, will give the specific gravity of the gas, by reference to the following Table and Rule: RULE.-Deduct the counter-weight from the weight in the table opposite to the temperature and barometric pressure, or the nearest there given; divide the remainder by the tabular weight, and the first three figures in the quotient will be the specific gravity, or the proportion which the gas bears to 1000 of air. As the differences of aqueous vapour are so inconsiderable within the range of oil and coal gases, especially under the circumstances in which this operation is performed, it is not thought necessary to include them. Table of the weight of 50 cubic inches of air, at different temperatures and barometric pressures. 100 inches being token at 30.5 grains. Pressure 30. Temperature 60°. BAR. In.Tnths 28.0 28.1 282 28.3 28.4 28.5 28.6 28.7 28.8 28.9 29.0 29.1 $29.2 29.3 29.4 29.5 29.6 29.7 THEK. 32° | 36° | Grains. 40° | 45° | 50° | 55° | 60o. | 6.5° 15.06 14.94 14 81 14.66 14.52 14.37 14.23 14.09 15.12 | 14.99 | 14.87 15 65 14.72 | 14.57 | 14.42 | 14.28 | 14.14 14.77 | 14.62 | 14.48 | 14.33 | 14.19 | 15.17 | 15 04 | 14.92 | | 15.22 | 15.10 | 14 97 | 14.82 | 14.67 | 14.53 | 14:38 | 14.24| 15.28 15.15 | 15.03 | 14.87 | 14.72 | 14 58 | 14.43 | 14.29 15.33 | 15.20 | 15.08 | 14.93 | 14.78 | 14.63 | 14.49|14.34| 15.38 | 15.26 | 15.13 | 14 98 | 24.83 | 14.68 | 14.54 | 14.40 15.44 | 15.31 | 15.19 | 15.03 | 14.88 | 14.73 | 14.59 | 14 45| 15.49 | 15.36 | 15.24 | 15.08 | 14.93 | 14.78 | 14.64 | 14.50| 15.55 | 15.42 | 15.29 | 15.14 | 14 98 | 14.83 14.69 14.55 15.60 | 15.47 | 15.34 | 15.19 | 15.04 | 14.89 | 14.74 | 14.60| 15.52 | 15.40 | 15 24 | 15.09 | 14.94 | 14.79 | 14.65 | 15.71 | 15.58 | 15.45 | 15.29 | 15.14 | 14.99 | 14.84 | 14.70 15.76 | 15.63 | 15.50 | 15.35 | 15.19 | 15.04 | 14.89 | 14 75 15.81 | 15.68 | 15.56 | 15.40 | 15.24 | 15.09 | 14.94 | 14.80 15.87 15.74 | 15.61 | 15.45 | 15 30 | 15.14 | 14.99 | 14.85 | 15.92 | 15.79 | 15.66 | 15.50 | 15.35 | 15.19 | 15.05 | 14.90 15.98 15.84 | 15.71 | 15.56 | 15.40 | 15.25 | 15.10 | 14.95 29.8 | 16.03 15.90 | 15.77 | 15.61 | 15.45 | 15.30 | 15.15 | 15.00 | 16.08 | 15.95 | 15.82 | 15.66 | 15.50 | 15.35 | 15.20 | 15.05| 16.14 16.00 15.87 | 15.71 | 15.55 | 15.40 | 15.25 | 15,10 16.19 | 16.06 | 15.93 | 15.76 | 15.61 | 15.45 | 15.30 ↑ 15.15| 30.2 |_ 16.25 | 16.11 | 15.98 | 15.82 | 15.66 | 25.50 | 15.35 | 15.20 30.3 | 16.30 | 16.16 | 16.03 | 15.87 | 15.71 | 15.55 | 15.40 | 15.25 16.35 | 16.22 | 16.08 | 15.92 | 15.76 | 15.60 | 15.45 15.30 16.41 | 16.27 | 16.14 | 15 97 | 15.81 | 15 66 | 15.50 | 15 35 16.46 16.32 | 16 19 | 16.03 | 16.87 | 15.71 | 15.55 | 15.40| 16.51 | 16.38 | 16.24 | 16.08 | 16.92 | 16.76 | 15.60 | 15.45 16.57 | 16.43 | 16.30 | 16.13 | 16.97 | 15.81 | 15.66 | 15.50 16.62 | 16.48 | 16.35 | 16.18 | 16.02 | 15.86 | 15.71 | 15.55| | 16.68 | 16.54 | 16.40 | 16 24 | 16.07 | 15.91 | 15.76 | 15.60 29.9 30.0 30.1 30.4 30.5 30.6 30.7 30.8 30.9 31.0 28.1 14.01 28.2 1 28.3 1 28.4 1 28.5 1 28.6 28.7 1 28.8 28.9 29.0 14.46 29.1 14.51 29.2 29.3 14.60 13.82 | 13.87 | 13.74 | 13.61 | 13.49 | 13.36 | | 14.61 | 14.47 | 14.34 | 14.21¦ 14.07 | 13.95 13.99 14.04 14.09 14.14 14.18 14 23 14.28 14.32 15.21 | 15.06 | 14.92 | 14.78 | 14.65 | 15.40 15.26 14.51 15.11 | 14.97 | 14.83 | 14.69 | 14.56 15.16 | 15.02 | 14.88 | 14.74 | 14.61 OF THE MAINS, SYPHONS, SERVICE-PIPES, The large cast-iron pipes by which the gas passes from the gasometer through the range of streets to the different points where it is consumed, are denominated the gas mains, and the arrangement and proper laying down of these probably require as much attention and consideration as any branch of the art of Gas-lighting. The ingenuity and skill of the gas engineer are here particularly called into exercise, because, in large towns, the difference in the levels of the respectire places to be lighted is very great, and he has therefore to take into account their relative height and distance, as well as the quantity of gas to be supplied, so that he may adapt the mains for affording an adequate supply wherever it may be wanted. The mains vary in their diameter from sixteen to two inches; the largest are generally placed the nearest to the gas works, and in the middle of the streets; but their sizes are varied according to circumstances, such as the elevation, or distance of the place, the number of lights, &c. Where the streets are wide, and the number of |