ADDRESS OF PROF. A. GRAY. 63 



He understood, as few do, the importance of analogy and hypoth- 

 esis in science. Premising that hypothesis should always be founded 

 on real analogies and used interrogatively, he commended it as the 

 prerequisite to experiment, and the instrument by which, in the 

 hands of sound philosophers, most discoveries have been made. This 

 free use of hypotliesis as the servant and avant-courier of research — 

 as means rather than end — is a notable characteristic of Henry. 

 His ideas on the subject are somewhat fully and characteristically 

 expounded by himself in his last presidential address to the Philo- 

 sophical Society of Washington, — one which he evidently felt 

 would be the last. 



How Henry was valued, honored, revered at Princeton, the 

 memorial published by his former associates there feelingly declares. 

 What he did there for science in those fourteen years would be long 

 to tell and difficult to make clear without entering into details, 

 here out of place. Happily the work has been done to our hand 

 by the Professor himself, in a communication which is printed 

 in the index volume of the Princeton Review, and reprinted in tho 

 Princeton Memorial. 



One of these, of the Princeton period, ought to be mentioned. 

 It is upon the origin of mechanical power and its relations to vital 

 force. It is a characteristic example of Professor Henry's happy 

 mode of treating a scientific topic in an untechnical way. It also 

 illustrates his habit of simply announcing original ideas without 

 putting them prominently forward in publication, as any one who 

 was thinking of himself and of his own fame would be sure to do. 

 The doctrine he announced was communicated to the American 

 Philosophical Society in 1844 in brief outlines. He developed it 

 further in an article published in the Patent Office Report for 1856, 

 twelve years later; a medium of publication which was naturally 

 overlooked. Only at a friend's desire was the paper reproduced, 

 in 1860, in the American Journal of Science, where it would be 



