(34.)
In effecting this, steam of a pressure equal to three times that of the atmosphere acts on the inner surface of the vessels V V′. One third of this bursting pressure is balanced by the pressure of the atmosphere on the external surface of the vessels; but an effective pressure of thirty pounds per square inch still remains, tending to burst the vessels. It was found that the apparatus could not be constructed to bear more than this with safety; and, therefore, in practice, the lift of such an engine was limited to about ninety perpendicular feet. In order to raise the water from the bottom of the mine by these engines, therefore, it was necessary to place one at every ninety feet of the depth; so that the water raised by one through the first ninety feet should be received in a reservoir, from which it was to be elevated the next ninety feet by another, and so on.
Besides this, it was found that sufficient strength could not be given to those engines, if constructed upon a large scale.
They were, therefore, necessarily very limited in their dimensions, and were incapable of raising the water with sufficient speed. Hence arose a necessity for several engines at each level, which greatly increased the expense.