SAVERY'S ENGINE. — BOILERS AND THEIR APPENDAGES. — WORKING APPARATUS. — MODE OF OPERATION. — DEFECTS OF THE ENGINE. — NEWCOMEN AND CAWLEY. — ATMOSPHERIC ENGINE. — ACCIDENTAL DISCOVERY OF CONDENSATION BY INJECTION. — HUMPHREY POTTER MAKES THE ENGINE WORK ITSELF. — ADVANTAGES OF THE ATMOSPHERIC ENGINE OVER THAT OF SAVERY. — IT CONTAINED NO NEW PRINCIPLE. — ITS PRACTICAL SUPERIORITY. (31.) The steam engine contrived by Savery, like every other which has since been constructed, consists of two parts, essentially distinct. The first is that which is employed to [Pg050] generate the steam, which is called the boiler; and the second, that in which the steam is applied as a moving power. [Illustration: _Fig._ 11.] The former apparatus in Savery's engine consists of two strong boilers, sections of which are represented at D and E in _fig._ 11.; D the greater boiler, and E the less. The tubes T and T′ communicate with the working apparatus, which we shall presently describe. A thin plate of metal R, is applied closely to the top of the great boiler D, turning on a centre C, so that by moving a lever applied to the axis C on the outside of the top, the sliding plate R can be brought from the mouth of the one tube to the mouth of the other alternately. This sliding valve is called the _regulator_, since it is by it that the communications between the boiler and two steam vessels (hereafter described) are alternately opened and closed, the lever which effects this being moved at intervals by the hand of the attendant. Two _gauge cocks_ are represented at G, G′, the use of which is to determine the depth of water in the boiler. One, G, has its lower aperture a little above the proper depth; and the other, G′, a little below it. Cocks are attached to the upper ends G, G′, which can be opened or closed at pleasure. The steam collected in the top of the boiler pressing on the surface of the water, forces it up in the tubes G, G′, if their lower ends be immersed. Upon opening the cocks G, G′, if water be forced from both, there is too much water in the boiler, since the mouth of G is _below_ its level. If steam issue from both, there is too little water in the boiler, since the mouth of G′ is _above_ its level. But if steam issue from G, and water from G′, the water in the boiler is at its proper level. This ingenious contrivance for determining the level of the water in the boiler is the invention of Savery, and is used in many instances at the present day.