106 MEMORIAL OF JOSEPH HENRY. 



he know when to sow and when to harvest; would the mariner 

 have signals of danger and the merchant, warrior, and diplomat 

 messages as fleet as thought; the knowledge of this philosophic 

 mind rallied to its work, with a zeal which never flagged, and a 

 practical success beyond all expectations and praise. And thus in 

 various branches of physics he was the companion of HARE, SILLI- 

 MAN, DRAPER, TORREY, AGASSIZ, GUYOT, GRAY, PEIRCE, 

 BACHE, and BAIRD; the student of NEWTON, CUVIER, ARAGO, 

 WOLLASTON, and others of perpetual fame; and the correspondent 

 of FARADAY, TYNDALL, PROCTOR, and others of another hemi- 

 sphere who are engaged in active, daily, arduous duty to science. 



In a tractate which he wrote in December, 1876, concerning his 

 researches while at Princeton, he gives a most interesting account of 

 his contribution with reference to the origin of mechanical power 

 and the nature of vital force. How plainly he defined and how 

 richly he colored this recondite subject! He takes the crust of the 

 earth in a state of equilibrium and describes the substances which 

 constitute that crust, such as acids and bases. He pursues them 

 into a state of permanent combination, inert and changeless. True, 

 he finds what he calls an infinite thin pellicle of vegetable and 

 animal matter on the surface men and mollusks, Caucasians, con- 

 gressmen, and conifers, elephants, and forests; but all the changes 

 on that surface he refers to a beautiful law of light radiating from 

 celestial space! How comprehensively he generalizes all the prime 

 movers which produce molecular changes in matter ! 



These he refers to two classes : the first, that of water, tide, and 

 wind power; the second, steam and other powers developed by 

 combustion, and animal power. Gravity, cohesion, electricity, and 

 chemical attraction, while they tend to produce a state of equilibrium 

 or repose on our planet, are only secondary agents in producing 

 mechanical effects. Must not the water have its level on the surface 

 of the ocean? In seeking it, is it not a force for the welfare of 



