The simplest and best of these is the common gasometer, consisting of a cylindrical or prismatic vessel open at the bottom, and suspended over wa ter by means of a lever or pulleys, with chain and counterbalance weights; a machine with which che mists have been familiar since the days of Lavoisier, with whom it appears to have originated, and who y.blished a description of it in 1789.
Watt's air-holder, for containing inflammable or other airs or gases, though known to the public for upwards of twenty-five years, has now been made the subject of a patent, as applicable to the pur poses of a coal gas reservoir, but for which it is not at all suited, however appropriate to the originally intended purposes of containing gases, and transport ing them from one place to another.
It has been proposed to diminish the size of the vessels used for containing gas, by forcing it - to them in a greatly compressed state; but unless the gas-holders for this purpose are made enormously strong, any great degree of compression cannot be attained with safety. Small portable vessels, taining gas in a compressed state, have been used for supplying light ; one of these was exhibited at the Royal Institution in 1816 or 1817 (Quarterly equalizing the action of the gasometer and counter poise as opposed to each other, is less complete than the following: Let the counterpoise (instead of be ing formed as shown at e in the figure) be allowed to descend into the water, and consist of a long cylin drical or prismatic body as represented by the dots g g, having the area of its horizontal section equal to the area of a similar section of the plates, or sub stance of which the gasometer is formed ;. and let the chain d be of a weight equal (length for length) to a column of water of equal bulk with the counter poise.
lbs.
For instance, let the gasometer, as before, weigh, in . . . 1000 And, in water, say,. 950 The counterpoise also to weigh, in air, 1000 And, when immersed, like the gasometer, 950 And, a portion of the chain equal in length to the space through which the gasometer rises or falls, 50 .
Then, when the gasometer is immersed in water its effective weight is. 950 To which must be added the chain now assisting it as weight, . 50 Sum, corresponding with the weight of low the counterpoise, Reversing the case, let the gasometer be out of water, it then weighs. 1000 The counterpoise now immersed, its effec tive weight is. 950
And the chain, now assisting the same, 50 Gives a sum of 1000, corresponding to the weight of the gasometer ; and in every point of the ascent and descent, this oppo site equality will be found to exist.
It will have occurred to the reader, that gasome ters counterpoised in the ways above described, can have no power to expel their contents ; but it is ob vious enough that the counterpoise may be lessened in weight so as to cause any given pressure on the gasometer, but without affecting the equality of such pressure.
Were the gases made use of for illumination, of the same specific gravity with atmospheric air, the above method of adjustment would be perfect, but as the specific gravity of coal gas is considerably less than that of the air (being only about two-thirds of it at the common atmospheric pressure), a com pensation for this is requisite. The gasometer, when filled with such gas, will, of course, require a less weight to counterbalance it than it otherwise would if filled with air, and the weight of the chain must be therefore lessened as will now appear.
Let it be supposed (in consequence of the le- lbs. vity of the gas) that the gasometer, when fill ed, weighs, in effect, only 995 Then the counterpoise must actually weigh 995 and its horizontal section must be lessened, so as to displace, of water, only 45 lbs.; and the portion of chain above mentioned must weigh only 45 lbs. It will then be found, that, as in the former case, an equilibrium subsists between the gasometer and counterpoise.
We have seen, where a sufficient depth of cistern was not obtainable, a form of gasometer, represent ed in fig. 2, adopted. This consists of two parts, de tached from each other ; the inner one, a a, being of itself a gasometer of the common construction, but surrounded with a channel, 6 b, containing water, and which, as it rises up, connects with the outer part, c c, and carries it upwards also ; the two form ing together one gasometer. In like manner, more outer parts might be added, but the thing is sufficient ly complex, as shown ; d d are pulleys, with weights just sufficient to counterbalance the outer part, c c, but not to elevate it without the assistance of the weight, e. Other contrivances have been proposed for saving of room on somewhat similar plans, but, like the above, they are not deserving of much atten tion, and only proper to be resorted to in cases of necessity.