326 Mr. E. B. Rosa on the Deter mi nation ofv, the 



raked with a wire, to prevent error from small air-bubbles 

 which tended to lodge there), and weights again added to 

 balance. The following weighings were thus made: — 



130-7 g. 130-5 g. 131-2 g. 131*3 g. Mean = 131-4. 

 132-0 g. 1320 g. 131-7 g. 131*4 g. Mean temp., 18°*4. 

 131-6 g. 131-4 g. 



The condenser being emptied and carefully dried, required 

 the following weights to balance the same counterpoise: — 



8650-3 g. 8650*8 g. 8650-9 g. 8650*9 g. Mean 8650*7 g. 

 8650*7 -l&l*4=8519-3= weight of water at 18°*4 in air. 

 8531-8= „ „ 4°-0 „ 



Correction for dis- 



placed air 



:} 



8540*9= „ „ „ in vacuo 



= volume of the shell in cubic centimetres. This makes the 

 radius r= 12*6805 cm. It seems reasonable to suppose that 

 the number 8540*9 is true to within less than a gram. This 

 would make the error in r less than *0005 cm. 



These values of the radii are confirmed by the following 

 direct measurements, made on a dividing-engine, using 

 calipers and a standard metre-bar by Bartels and Diederichs, 

 Gottingen, whose length is accurately known. Three mutually 

 perpendicular diameters of the shell were found to be 



25*357, 25*360, 25*358. Mean = 25*3583, 



giving t — 12*6791, a very close agreement in view of the 

 difficulty of setting the calipers. More accurate measure- 

 ments on the balls were obtained. 



Ball A. — Following are twelve diameters : — 



20*2399 20*2372 20*2170 



20*2358 20*2336 20*2348 



20*2350 20*2382 20*2250 



20*2250 20*2315 20-2401 



Mean = 20'2328, + Correction for the bar -0038, =20*2366 ; 

 .*. r' A = 10*1183 cm. 



Ball B. — Following are six diameters : — 



17-7468 17-7408 17*7429 



17*7465 17*7452 17*7407 



Mean = 17*7438, + Correction *0034, =17*7472 ; 



.-. r' = 8*8736 cm. 



