820 Transactions of the American Institute. 



called the Allen engine, which was pronounced by them in their 

 report above mentioned, to be first in the order of merit. The 

 opinion of the judges here referred to was founded upon a careful 

 testing of the performance of the engine and an examination of its 

 construction ; but it was announced as a simple judgment upon the 

 facts, without any attempt to connect these with the theoretic prin- 

 ciples to which, according to the claim of the exhibitors, the excel- 

 lence of the performance is due. 



But it was' upon the soundness of these principles chiefly that 

 these gentlemen had been desirous to obtain a verdict. In conse- 

 quence, therefore, of their solicitations, frequently renewed after the 

 closing of the fair, the investigations, of which the results are pre^ 

 sented below, have been recently undertaken ; and in compliance with 

 a written request addressed to the undersigned by Mr. Charles T. 

 Porter, one of the exhibitors, under date of February 15, 1872, these 

 results are here embodied in the form of a supplementary report. 



It may be remarked of this engine, in the beginning, that the 

 principal object aimed at in its construction is to secure steadiness 

 of action and uniformity of strain upon the crank shaft, while work- 

 ing steam with large expansion ; and to accomplish this in such a 

 manner as to make the engine practically its own regulator, and to 

 render it in a great degree (it cannot be entirely) independent of the 

 fly-wheel. It has other noticeable peculiarities ; as, for instance, its 

 variable cut-off, operated by the link motion, without which it proba- 

 bly could not be run advantageously with the high velocities which 

 its theory requires ; and its admirably balanced valves, in which the 

 effects not only of pressure but of current have been considered, 

 and which give prompt and free admission to the steam even when 

 the cut-off is short ; but these are features of secondary importance 

 on which it is not necessary to dwell in this place. 



To maintain steady action under very largely varying pressure of 

 steam upon the piston, is, then, the main problem which the con- 

 structors of this engine have set themselves to solve. The mechani- 

 cal solution which they offer is simple, but it is opposed to notions in 

 regard to reciprocating engines which have long been traditionally 

 received, and, therefore, it should be closely criticised. 



This solution consists in giving to the reciprocating parts of 

 the engine a very considerable weight, so that, as the pressure 

 of the steam falls off during the latter part of the stroke, 

 their inertia may cause them to act as a driving power instead of 

 it. The living force which these moving parts will embody will be 



