energy_, yet if we attached a thermometer to the drop we would not find that it was any warmer than before the drop was raised. If it were possible to make an instrument sufficiently sensitive to indicate an instantaneous change of temperature in the weight while falling, we would not find any increase of its temperature at the instant it had acquired its greatest momentum and just before it struck the object under it, although its potential energy would at that instant be converted into _actual energy_ of motion. If, however, the weight should strike an unyielding object, its actual energy would at once be reconverted into heat, which our thermometer would indicate. The phenomenon of what is called latent heat of evaporation seems to be very similar to that described--the heat when the water is changed from a liquid to a gaseous condition is transformed into energy, which, as already stated, has no effect upon the mercury of the thermometer. [11] Balfour Stewart on the Conservation of Energy. QUESTION 40. _What is meant by the_ TOTAL HEAT _of steam?_ _Answer._ The “total heat of steam” is a phrase used to denote the sum of the heat required to raise the temperature of water from some given point up to the boiling-point due to a given pressure, and of the heat which disappears in evaporating one pound of water under a given pressure (or _latent heat of evaporation_.) Thus the latent heat of one pound of steam at atmospheric pressure (14.7 lbs.) is 966.1 units; and 212 units of heat are necessary to raise water from zero to the boiling-point; therefore the total heat counted from zero of steam of atmospheric pressure is 1,178.1 units. At 100 pounds absolute pressure the latent heat is 885.5 and the sensible heat 327.9 degrees; therefore the total heat measured from zero is 1,213.4 units. PART IV. THE SLIDE-VALVE. QUESTION 41. _What are the essential conditions which a slide-valve must fulfill in governing the admission and exhaust of steam to and from the cylinder of an ordinary engine?_ _Answer._ 1. It must admit steam to one end only of the cylinders at one time. 2. It must allow the steam to escape from one end at least as soon as it is admitted to the other end; and 3. it must cover the steam-ports so as not to permit the steam to escape from the steam-chest into the exhaust-port. [Illustration: _Fig. 10._ Scale ³⁄₁₆ in. = 1 inch.] QUESTION 42. _What was the first form of slide-valve used?_ _Answer._ That represented in fig. 10. The smallest movement of this valve either way opens one of the steam-ports for the admission of steam and puts the other in communication with the exhaust-port. By cutting a piece of ordinary writing paper to the form of the section of the valve, and moving it on the line _f f_, the action of the valve will be clearly shown. [Illustration: _Plate I._ AMERICAN LOCOMOTIVE. By The Grant Locomotive Works, Paterson, New Jersey. _Scale, ³⁄₈ in. = 1 foot._]