1876.] A. Pedler — On the use of the Radiometer as a Fliotometer. 189 



lllmninating poiver of Gas-jet, in 

 standard sperm candles hurn- 

 inrj 120 grains per hour. 



1. 



1234 



2. 



12 04 



3. 



1010 



•4. 



11-48 



5. 



12-42 



Nii/inher of qitarter-revolutions of 

 'Radiometer per minxde (at 

 27*2 inches distance). 



35-7 

 40-0 

 29-0 

 380 

 35 



It will be seen that there .is here a general sort of agreement between 

 the number of revolutions and the illuminating power, but that in one or 

 two instances discrepancies occur. I therefore thought it better to extend 

 the observations of the radiometer by altering the distances from the source 

 of lio-ht. By doing this, it would also prove whether the mechanical effect 

 produced could be brought under any definite law. I therefore arranged 

 that the radiometer could be placed at the distances 10, 15, 20, 25 and 

 30 inches respectively from the gas-jet, and made a series of observations 

 of the rapidity of revolution, two and, in many cases, three measurements 

 at each distance being taken. The results are given in the table below, 

 and it will be again seen, that there are discrepancies between the illumina- 

 ting power and the observed revolutions, and that also these discrepancies 

 extend throughout the observations at the varying distances. 





lUuTTiinating 

 power of 



Di^ 



tance of r 



adiometer fi-om g-as 



jet. 





lu inches 



15 inches. 



20 inches. 



25 inches. 



30 inches. 





gas-jet. 













- 



12-42 



135 



79-3 



44-25 



26-67 



19-67 





10-58 



162 



108-5 



66 



45-5 



27 





13-12 



162 



121-5 



75 



43-5 



29 





9 53 



152 



97 



60 



38 



25 





13-42 



154-67 



104-5 



64-5 



45-5 



29 





1306 



170-5 



109-5 



66 



43-5 



31-5 





11-86 



163 



112 



67 



46 



34 



Average, 



12-00 



157-02 



104-57 



63-25 



41-25 



26-45 







Practical .result, calculated to the rate 













of 10 per minute at 30 inches distance 



59-36 



39-53 



23-91 



15-59 



10-0 



Theoretical result, calculated according 













to law of inverse squares, 



90 



40-0 



22-5 



14-4 



10-0 



At the bottom of the table I have calculated the observed rate of re- 

 volution, starting with a supposed unit of 10 quarter-revolutions, at the 

 distance of 30 inches ; and it will be scon that these results agree very 

 closely with those calculated according to the law of inverse squares ; that 

 is to sa}'', the number of revolutions ofc" the radiometer will be inversely 



