In an article published on page 105 of the last volume, we endeavored to show that whatever work was performed by the mass motion of a steam engine was a subtraction from that mass motion, the mass motion itself being a subtraction from the heat of the steam. Some correspondents have either failed to recognize the truth of the proposition laid down in the article referred to, or they do not understand it The following query will illustrate: " Why will not the exhaust steam from an engine impart to water the same amount of heat it contained when it entered the cylinder, minus that radiated from the cylinder during the stroke f This correspondent has been experimenting as follows: He estimates from standard formulae the amount of heat in the steam entering the cylinder of his engine at each stroke, and the radiaticn irom his cylinder during the stroke, and finds that the number of units of heat imparted to the water used for condensation, falls short of the amount contained in the steam when admittted to the cylinder, far more than can be accounted for by radiation. He is surprised at the result, but has brought himself by this experiment to the threshold of a discovery, none other than the law of the correlation and conservation of force. The reason for his undertaking the experiment alluded to is instructive. He desired to heat his building with the same fuel required to drive his engine, supposing that as the heat escaping from the boiler and engine helped to warm the air of his factory, he might capture all the rest by passing the exhaust through some combination of pipes or heaters, and thus use the entire heat generated by the combustion of the fuel consumed for heating purposes, making a clear gain of the work performed. His experiments have convinced him that this can not be done but he fails to see why. The reason is, simply, that the heat which is converted into work can only be used for heating purposes when reconverted into heat. The correlation of forces, that is, their convertibility into each other, and the conservation of force, that is, its indestructibility, are two fundamental principles of physics. If the heat imparted to steam by the combustion of fuel could be wholly converted into work, the steam would be wholly condensed and leave the exhaust in the form of water, its temperature being precisely that at which it was fed to the boiler. Buij. much of the heat escapes by radiation before any work is performed by the steam. This heat still exists as heat, and may be used to warm a building or for any other purpose to which it can be advantageously applied. The steam when it performs work becomes partially condensed in the effort; a portion of it is converted into mass motion or work, and only a residue escapes in the exhaust steam. So far as the mass motion is employed in overcoming friction, it becomes reconverted into heat, but when employnd to the raising of weights or to the overcoming of any resistance other than friction, it is not reconverted into heat but into other forces. If the coal used in the furnace of the engine be such an amount that it would exactly heat the building in which the engine is placed, and the radiated heat and that contained in the exhaust steam be applied directly to heating the building, it will be found that an additional expenditure of coal proportional to the amount of the work performed by the engine, will be needed to secure the proper warmth, not taking into account the escape of heat through the smokepipeor chimney which is always a source of loss.