Excerpt from A Catechism of the Steam Engine: Illustrative of the Scientific Principles Upon Which Its Operation Depends, and the Practical Details of Its Structure, in Its Application to Mines, Mills, Steam Navigation and Railways; With Various Suggestions of Improvement A. - Because it is not the vacuum which exerts this pressure, but the atmosphere, which, like a head of water, presses on every thing immerged beneath it. A head of water, however, would not press down a piston, if the water were admitted on both its sides; for an equilibrium would then be established, just as a balance retains its equilibrium though an equal Weight be added to each scale; but take the weight out of one scale, or empty the water from one side of the piston, and motion or pressure is produced; and in like manner pressure is produced on a piston by admitting steam or air upon the one side, and withdrawing the steam or air from the other side. It is not, therefore, to a vac uum, but rather to the existence of an unbalanced plenum, that the pressure made manifest by exhaustion is due, and every one knows that a vacuum of itself would not work an engine. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.