From the foregoing it will be readily understood that if the steam is allowed to exhaust from the cylinders at or near the pressure at which it is admitted the work which it might have accomplished by expansion will be lost. This means not only a loss of the steam but of a part of the fuel used to generate it. It is therefore advisable to get all the work out of the steam that is possible and the nearer to atmospheric pressure the exhaust can be brought the greater will be the economy. [Footnote A: NOTE.—This unit has been recently changed to 778.] USEFUL DEFINITIONS RELATING TO STEAM. _Steam_ is water in a gaseous state; the gas or vapor of water; it liquifies under a pressure of 14·7 and temperature of 212° F. _Steam_ is a joint production of the intermingling of water and heat. Water is composed of two gases which have neither color nor taste, and steam is made up of the same two gases with the addition only of that mysterious property called heat by which the water becomes greatly expanded and is rendered invisible. The French have a term for steam which seems appropriate when they call it water-dust. This is what takes place in the formation of steam in a vessel containing water in free communication with the atmosphere. At first, a vapor is seen to rise that seems to come from the surface of the liquid, getting more and more dense as the water becomes hotter. Then a tremor of the surface is produced, accompanied by a peculiar noise which has been called _the singing_ of the liquid; and, finally, bubbles, similar to air bubbles, form in that part of the vessel which is nearest to the fire, then rise to the surface where they burst, giving forth fresh vapor. The curious fact must be here noted that if water be introduced into a space entirely void of air, like a vacuum, it vaporizes instantaneously, no matter how hot or cold, so that of an apparent and fluid body there only remains an invisible gas like air. That steam is _dry_ at high pressure is proved by an experiment which is very interesting. If a common match head is held in the invisible portion of the steam jet close to the nozzle, it at once lights, and the fact seems convincing as to complete dryness, as the faintest moisture would prevent ignition even at the highest temperature. This experiment proves dryness of the steam at the point of contact, but if throttling exists behind the jet, the steam supplied by the boiler may be in itself wet and dried by wire drawing. _Dead steam_ is the same as exhaust steam. _Live steam_ is steam which has done no work. _Dry steam_ is saturated steam without any admixture of mechanically suspended water. _High-pressure steam_ is commonly understood to be steam used in high pressure engines. _Low-pressure steam_ is that used at low pressure in condensing engines, heating apparatus, etc., at 15 lbs. to the inch or under.