34 ENGINE TESTS. 



clearance space with water which is poured into the open port 

 through a funnel. The water is drawn from a receptacle contain- 

 ing a sufficient quantity, and this has previously been measured. 

 When the whole space, including the port, is completely filled, 

 the quantity left is measured, and the difference shows the 

 amount that has been poured in. The measurement can be 

 most easily made by weighing the water, and the corresponding 

 volume determined by calculation. The proportion required in 

 the formula is the volume in cubic inches thus found, divided 

 by the volume of the piston displacement, also in cubic inches, 

 and the result expressed as a decimal. 



The only difficulty which arises in measuring the clearance 

 in this way is that occurring when the exhaust valve and 

 piston are not tight, so that, as the water is poured in, it flows 

 away and is lost. If the leakage is serious, no satisfactory 

 measurement can be made, and it is better to depend upon the 

 volume calculated from the drawing. If not too serious, how- 

 ever, an allowance can be made by carefully observing the 

 length of time consumed in pouring in the water ; then, after 

 a portion of the water has leaked out, fill up the space again, 

 taking the time and measuring the quantity thus added, deter- 

 mining in this way the rate at which the leakage occurs. Data 

 will thus be obtained for the desired correction. 



In the tests here reported the clearance has not, as a rule, 

 been determined by actual measurement in the manner noted, 

 nor even in all cases by the calculation from the drawing. In 

 cases where the proportion of clearance is assumed, the assump- 

 tion is based on the known clearance of similar classes of engines, 

 determined either by water measurement or calculation. The 

 effect which a small error in the clearance may have upon the 

 result of the computation of steam accounted for is not of a 

 serious nature, unless it is a case where the cut-off is very short. 

 For example, if the steam accounted for with a clearance of five 

 per cent comes out T W of the feed-water consumption, the re- 

 sult with a clearance of 4 % would be T 7 o 3 ^, changing the pro- 

 portion about T 7 at cut-off and much less at Telense. 



In compound and other multiple expansion engines the same 



