GASOMETERS. 135 



The figures at the side of the tank in the engraving refer to the thickness of 

 metal at that part opposite to which they are placed. 



In any vessel containing a heavy fluid, the parts that are deepest below the 

 surface sustain a proportionably greater pressure ; in the construction of the 

 tank, therefore, we should run into superfluous expense by making the sides 

 equally thick in every part ; for if the substance be uniformly thick, and the 

 lower parts are sufficiently strong, the upper parts are consequently much 

 more so than necessary. The method suggested by theory is, while we give 

 to the whole tank the same interior diameter, to give a safe and sufficient 

 thickness at the lower part, and let it gradually diminish to the top, in the 

 same ratio nearly as the diminution in the depth of the fluid. But in prac- 

 tice we must vary the construction ; for although the plates of which the 

 tank is composed, taken separately, may be sufficiently strong to resist the 

 pressure of the water, yet, taken collectively, their thicknesses must depend in 

 a great measure upon the strength required at the joints ; therefore iron hoops 

 are added to the two lower tiers of plates, to make up the difference in strength 

 between the upper and lower tiers, instead of increasing the thickness of metal 

 at these parts, and adding to the expense. 



It may be as well also to say, that in bolting the plates together they must 

 " break joint," as represented in the figure. 



The plates forming the bottom are three quarters of an inch thick, and joined 

 in the same way as those at the sides. 

 B B is the lower division or slide of the gasometer, furnished at the upper part 



with a returned rim, b, about twelve inches deep, and three inches wide 



from the side. 

 C C is the upper division, having at its lower edge a corresponding rim at b, 



but reversed ; so that when in action it has risen to its full height, the 



returned rim of the lower division will dip into the water contained in 



that of the lower one, and form an hydraulic joint. 



In the action of this gasometer it is evident that the upper portion must rise 

 first, and having attained the proper altitude, will, as it were, unite itself with 

 the lower portion, when they will both rise together : the whole vessel is guided 

 by rollers, similar to those used in ordinary gasometers ; but in addition to 

 these, it is found necessary, for the greater security of the upper portion, to 

 use standards and balance-weights, because the perpendicular height being 

 great, the vessel would otherwise have a tendency to work sideways, or bind ; 



