TANKS AND GAS-HOLDERS.

235

together with flanges and iron cement in the usual way, as shown in Fig. 80, which isdrawn to a scale of three-quarters of an inch to a foot. The figures at the side of thetank in the Plate refer to the thickness of the metal in those parts opposite to which theyare placed.

In any vessel containing a heavy fluid, the parts that are deepest below the surfacenecessarily sustain a proportionally greater pressure. In the construction of the tank there-fore 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 upperparts 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 sufficientthickness at the lower part, and let it gradually diminish to the top, in the same ratio nearlyas the diminution in the depth of the fluid. But in practice we must vary the construction ;for although the plates of which the tank is composed may be sufficiently strong to resist

Fig. 80.

the pressure of the water, yet taken collectively, their strength must depend in a great mea-sure upon the strength required at the joints : therefore iron hoops are added to the lower tiersof plates, to make up the difference in strength between the upper and lower tiers, instead ofincreasing the thickness of metal at those parts, and adding to the expense. It may be aswell also to say, that in bolting the plates together they must “ break joint,” as representedin the figure.

The plates forming the bottom are three-quarters of an inch, and joined in the same wayas those at the sides.

2 h 2