From the table it will be seen that one cubic foot of steam of 100 lbs. pressure weighs .2307 lbs.; therefore the weight of the fraction of a cubic foot given above would be calculated as follows: .2307 × 1608 ------------ = .2146 lb. = 1728 weight of 1608 cubic in. of steam of 100 lbs. absolute pressure. From the table it will be seen that the total heat above zero of steam of 100 lbs. absolute pressure is 1213.4 degrees. That is, as was explained in answer to Question 40,[19] in order to boil water under a pressure of 100 lbs. per square inch we must first heat water up to 327.9 degrees, and then, to convert it into steam, 885.5 degrees more must be added. It was also explained in the answer to Question 35 that one pound of water heated one degree is the standard of measurement or _unit of heat_. Now if we have 1 lb. of water with a temperature of zero, evidently it will take 1213.4 _units of heat_ to convert it into steam of 100 lbs. absolute pressure. But as the water from which our steam was generated had a temperature of 60 degrees, we must deduct that much from 1213.4: 1213.4 - 60 = 1153.4 = units of heat in one pound of steam of 100 lbs. absolute pressure generated from water of 60 degrees temperature. [19] In the illustration used in answer to Question 40, steam of 100 lbs. _effective_ pressure was used, whereas in the above case it is _absolute_ pressure. If now one pound of steam has 1153.4 units of heat, the following calculation will give the units of heat in .2146 lbs.: 1153.4 × .2146 = 247.51 = units of heat in .2146 lbs., or 1608 cubic in. of steam of 100 lbs. absolute pressure. It was shown in answer to Question 57 that the average pressure of steam of 100 lbs. cut off at 8 in. of the stroke was 69.95 lbs. per square inch. Disregarding the small fraction, we will call it 70 lbs. Now if we admit steam of this pressure through the _whole stroke_ of the piston, we will use 4,824 cubic inches. It will be found by a calculation similar to the above, that to generate this quantity of steam of 70 lbs. pressure from water of a temperature of 60 degrees would require 527 units of heat, or more than twice as many as were required to do the same work with steam of 100 lbs. pressure cut off at 8 inches when using it expansively during the rest of the stroke. The actual difference in practice is not so great as this, because the loss of heat from radiation and condensation in the cylinder and other causes is greater when steam of a high pressure is expanded than when lower pressure steam is admitted through the whole stroke. But after allowance is made for all such sources of loss and waste, there is still an enormous gain from using steam expansively. QUESTION 60. _What is meant by wire-drawn steam?_ _Answer._ It is the fall which the pressure of the steam undergoes during its passage from the boiler to the cylinder,[20] and which is due to the contracted opening of the steam pipes or valves. [20] Rankine.