HOW THE TESTS WERE CONDUCTED. 23 



The same method is usefully applied in tests of electric rail- 

 way engines. Indeed, except by some system of this kind, no 

 fair idea of the indicated horse-power can be obtained, and no 

 good comparison can be made between the indicated horse-power 

 and the electrical horse-power. In these engines it is best to 

 make the interval between the sets of diagrams thus obtained 

 not more than ten or fifteen minutes. It should be arranged 

 to give a signal every ten seconds while the operation is going 

 on, so that all the indicators may be worked together for the 

 three or four revolutions desired. Likewise, on the same signal 

 corresponding readings are taken of the electrical instruments. 

 This is continued until the period of time covered is two or 

 more minutes. The diagrams being all taken on the same card, 

 without unhooking the indicators, the means is at hand for ob- 

 taining an average for the whole period, as before pointed out. 



LEAKAGE TESTS OF VALVES AND PISTONS. 



The determination of the condition of an engine as to the 

 tightness of the valves and pistons has nothing to do with the 

 work of making a feed-water test, or of correctly ascertaining 

 the results. When, however, it comes to analyzing the results, 

 and ascertaining whether the engine is working with a proper 

 degree of economy, and if not, the reasons for the waste, it is 

 of the utmost importance that the matter of leakage should be 

 investigated. It is always desirable, therefore, when a feed- 

 water test is conducted, to supplement it by an inspection of the 

 valves and pistons having this object in view. This inspection 

 must be made when the engine is at rest. The conditions 

 which surround the internal working parts of an engine at rest 

 are entirely different from those of the engine in motion, and 

 for this reason it is held by some that an examination of leak- 

 age under these circumstances gives little information which 

 can be applied to working conditions. Those who take this 

 view hold that under conditions of motion the quantity of leak- 

 age is reduced, and it might happen that the leakage in motion 

 would be altogether insignificant, although very serious at rest. 

 The author takes the ground that the only course open in this 



