the absolute zero of temperature.  At one end erect an ordinate, upon which set off T = 3,777 deg., the temperature of the furnace.  At 849 deg. = t, on the scale of temperature, draw a line parallel to the base, and mark on it a length proportional to the heating surface of the boiler; join T by a diagonal with the extremity of this line, and drop a perpendicular on to the zero line.  The temperature of the water in the boiler being uniform, the ordinates bounded by the sloping line, and by the line, t, will at any point be approximately proportional to the rate of transmission of heat, and the shaded area above t will be proportional to the quantity of heat imparted to the water.  Join T by another diagonal with extremity of the heating surface on the zero line, then the larger triangle, standing on the zero line, will represent the whole of the heat of combustion, and the ratio of the two triangles will be as the lengths of their respective bases, that is, as (T — t) / T, which is the expression we have already used.  The heating surface was 220 square feet, and it was competent to transmit the energy developed by 41 lb. of coal consumed per hour = 12,819 u. x 41 u. = 525,572 units, equal to an average of 2,389 units per square foot per hour; this value will correspond to the mean pressure in an ordinary diagram, for it is a measure of the energy with which molecular motion is transferred from the heated gases to the boiler-plate, and so to the water.  The mean rate of transmission, multiplied by the area of heating surface, gives the area of the shaded portion of the figure, which is the total work which should have been done, that is to say, the work of evaporating 544 lb. of water per hour.  The actual work done, however, was only 485 lb.  To give the speculations we have indulged in a practical turn, it will be necessary to examine in detail the terms of Carnot’s formula.  Carnot labored under great disadvantages.  He adhered to the emission theory of heat; he was unacquainted with its dynamic equivalent; he did not know the reason of the difference between the specific heat of air at constant pressure and at constant volume, the idea of an absolute zero of temperature had not been broached; but the genius of the man, while it made him lament the want of knowledge which he felt must be attainable, also enabled him to penetrate the gloom by which he was surrounded, and enunciate propositions respecting the theory of heat engines, which the knowledge we now possess enables us to admit as true.  His propositions are: 

1.  The motive power of heat is independent of the agents employed to develop it, and its quantity is determined solely by the temperature of the bodies between which the final transfer of caloric takes place.

2.  The temperature of the agent must in the first instance be raised to the highest degree possible in order to obtain a great fall of caloric, and as a consequence a large production of motive power.