476 ' MEMORIAL, OF JOSEPH HENRY. 



To rightly appreciate a discoverer we should not look at his 

 work from our time, but go back and regard it from his time; we 

 should not judge his work in the fullness of the light of present 

 knowledge, but in the dim twilight which alone illuminated him to 

 then unknown but now well known facts and laws. I will there- 

 fore endeavor first to present you with a clear but necessarily very 

 concise view of the state of our knowledge of electricity when 

 Henry began his original researches in that branch of science, and 

 then point out the value of his discoveries by showing what they 

 added to knowledge and how they instigated and influenced the dis- 

 coveries and inventions of other men. 



Henry began his electrical researches at the age of 28, in the 

 year 1827, while he was Professor of Mathematics and Natural 

 Philosophy in the Albany Academy. At these he continuously 

 worked till 1832, when, at the age of 33, he moved to the College 

 of New Jersey (Princeton). After a year's break in his work, 

 caused by the preparation of his course of lectures for the college, 

 he is again at original research, and continues his contributions to 

 electrical discoveries till 1842. Thus, during 14 years, while be- 

 tween the ages of 28 and 43, he was a constant and fertile worker. 

 What he did in these years will be given after a review of what 

 had been already discovered up to the time he began his original 

 experiments. 



Through the labors of Gilbert, Boyle, Otto von Guericke, New- 

 ton, Wall, Gray, Franklin, .ZEpinus, and Volta, it had been dis- 

 covered that all matter could be electrically excited, and that bodies 

 differed greatly in permitting the diffusion of electricity over their 

 surfaces; the facts of electric attraction and repulsion, of electric 

 induction, the action of points, and the identity of lightning and 

 electricity had been discovered; and these facts had been explained 

 and bound together in a body of doctrine by the hypothesis of 

 Dufay or by that of Franklin; while Coulomb and Poisson, in a 

 series of beautiful experimental and mathematical labors, had given 

 us the knowledge of the laws of the actions at a distance of electric 

 attraction and repulsion, and had shown in what manner electricity 

 diffuses itself over conductors of various forms. 



