OV ARTS AND SCIENCES. 355 



^_. (n — 1) ax ^^ 



2 (2 ax-^ ga + gx) 



But C= ^^ 



•lb-^a-\-x 



(n — 1) ax ^ 



(2 ax-\^ga-\- gx) (2b-j-a-\-x) 



It is desirable to get a roiigli idea of the relation between work de- 

 rived directly from the energy in a given ray (as of sunlight), and that 

 coming mediately, through the effect upon the battery current modified 

 by the balance. A sunbeam one square centimetre in section, which 

 will, under the ordinary Allegheny sky, warm 1 gramme of water 

 1*^ C. in 1 minute or ^\° in 1 second, will raise the temperature of" a 

 sheet of water 1 mm. by 10 mm., or ^Lj of a square centimetre in area 

 and 3^Jij mm. thick, 83^-° C. in 1 second, if all the heat be retained ; and, 

 since the specific heat of platinum is .032, the same sunbeam will raise 

 a corresponding strip of platinum 2,603° C. in 1 second, it being sup- 

 posed here that there is no loss of heat by re-radiation, conduction, or 

 convection. This gives the startling result that the heat received from 

 ordinary sunshine on such a platinum strip would be sufficient to tJielt 

 it in less than a second, if it could all be retained, and is an independent 

 testimony to the rapidity with which these thin strips take up and part 

 with their heat, and to the promptness of action of the balance, whose 

 actual temperature is raised but a very few degrees by this radiation.* 



The heat produced in a strip of the above description by a current 

 of 1^ Weber is found to be (using C. G. S. notation), — 



(.012) X .5 X 109 



■^^ — ' = .00119 gramme degree per second, 



capable of raising the temperature of the strip 1,866° C. in 1 second if 

 all retained. A current of -^^j Weber raised a narrower iron strip 

 r2°.8 C. (see Table, p. 345). As in the first example it may be shown 

 by calculation that only a small fraction, in this case about ■^'^■^ of the 

 heat developed, is retained. 



It is intended eventually to enclose the Balance in a vacuum, and to 

 study more closely the losses by radiation which occur in it, apart from 



* The resistance of one arm of an Actinic Balance, exposed to radiation from 

 the sun at an altitude of 60° with an ordinary blue sky, was increased by an 

 amount which indicated tluit its temperature had been raised by 7° C. By cal- 

 culation it was found that the energy in the sunbeam would have been sufficient 

 {fall had been retained to heat the strips in 1 second about 450 times this amount. 



