OF ARTS AND SCIENCES. 343 



had flattered myself with the hope of succeeding better than my prede- 

 cessors. I found, however, that though I got results, they were too 

 obscure to be of any great value, and that science possessed no instru- 

 ment which could deal successfully with quantities of radiant heat so 

 minute. 



I have entered into these preliminary remarks as an explanation of 

 the necessity for such an instrument as that which I have called the 

 Bolometer (ySoA?;, /Aerpov), or Actinic Balance, to the cost of whose 

 experimental construction I have meant to devote the sum the Rum- 

 ford Committee did me the honor of proposing that the Academy 

 should appropriate. j 



Imi^elled by the pressure of this actual necessity, I therefore tried 

 to invent something more sensitive than the thermopile, which should 

 be at the same time equally accurate, — which should, I mean, be 

 essentially a " meter " and not a mere indicator of the presence of feeble 

 radiation. This distinction is a radical one. It is not difficult to make 

 an instrument far more sensitive to radiation than the present, if it is 

 for use as an indicator only ; but what the physicist wants, and what I 

 have consumed nearly a year of experiment in trying to supply, is 

 something more than an indicator, — a measurer of radiant energy. 



The earliest design was to have two strijis of thin metal, virtually 

 forming arms of a Wheatstone's Bridge, placed side by side in as 

 nearly as possible identical conditions as to environment, of which one 

 could be exposed at pleasure to the source of radiation. As it was 

 warmed by this radiation and its electric I'esistance proportionally in- 

 creased over that of the other, this increased resistance to the flow of 

 the current from a battery would be measured (by the disturbance of 

 the equaUty of the "bridge" currents) by means of a galvanometer. 



In order to test the feasibility of this method, various experiments 

 were made. To secure a radiating body which will not vary from one 

 experiment to another, or from day to day, is no easy matter. The 

 source employed during the preliminary trials has been commonly the 

 flame of a petroleum lamp within a glass chimney, the radiation being 

 limited by a circular opening of 1 cm. diameter in a triple cardboard 

 screen.* In these first trials a single thin metallic strip, being stretched 

 between appropriate metal clamps connected with the bridge by coarse 

 insulated wires, was enclosed in a cylindrical wooden case, which being 



* Very special precaution must be taken to prevent the screen itself from 

 getting heated. 



