HOW THE TESTS WERE CONDUCTED. 31 



The indicator cards relating to the tests reported here have, 

 as a rule, been measured by a polar planimeter. The average 

 obtained by going over the line of the diagram at least twice is 

 the reading taken. 



The mean effective pressure is determined by dividing the 

 scale of the spring by the length of the diagram expressed 

 in inches and decimals of an inch, and multiplying the quo- 

 tient by the area in square inches. The length of the dia- 

 grams, which is nearly constant, is found by selecting, say three 

 sets out of every ten taken on the test, and obtaining the 

 average length from those three. The horse-power is computed 

 by multiplying the "horse-power constant" for the cylinder 

 under consideration by the speed in revolutions per minute, and 

 by the mean effective pressure. The horse-power constant is 

 the power developed in the cylinder, assuming one pound mean 

 effective pressure and a speed of one revolution per minute. 

 It is obtained by multiplying the mean of the areas of the two 

 sides of the piston in square inches by twice the length of the 

 stroke in feet, and dividing the product by 33,000. The mean 

 effective pressure used is the mean of the two measurements 

 obtained at the two ends of the cylinder. In the detail tables, 

 giving the data and the results of the tests here reported, the 

 horse-power constant for each cylinder is given ; and the figures 

 of indicated horse-power in any case are the result of multipli- 

 cation of this constant, the revolutions given per minute, and 

 the mean effective pressure. For example, in the case of 

 Engine No. 1, which has a single cylinder 23" diameter, 5' 

 stroke, with one piston-rod 3^-' 7 in diameter, the horse-power 

 constant is .1247, the speed 74.7 r.p.m., and the m.e.p. 33.08 

 Ibs. The indicated horse-power, viz., 305.2, is the product of 

 these three quantities. 



The water per indicated horse-power per hour is found by 

 simply dividing the hourly consumption of water by the indi- 

 cated horse-power. In the example referred to, the hourly con- 

 sumption being 8477 Ibs., the feed-water per I.H.P. per hour is 

 8477 divided by 305.2 equals 27.77 Ibs. 



The method of determining the quantity of steam " accounted 



