616 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



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ment when it consisted of the heavy disk supported on the wire at the 

 same place as tluit formerly occupied by the light disk. The difference 

 between these decrements was that due to the resistance of the air on 

 the sphere alone. 



Description of Apparatus. 



Figure 1 shows the form of the appa- 

 ratus which was used. A is a glass globe 

 about 14 cm. in diameter to which, above, 

 is connected the tube, B, and below the 

 tube, C. The upper end of B is ground 

 to fit the cap, D, while the lower end of 

 C is ground to fit the mouth of the dry- 

 ing tube, E. The cap, D, is continued 

 above into the tube, F, through which a 

 brass rod surrounded by a rubber tube 

 can just pass. To the lower end of the 

 brass rod the fibre is attached. This 

 method of supporting the fibre enables 

 one to raise or lower the whole suspension 

 very easily. The fibre is about 0.0017 

 cm, in diameter, and about 35 cm. in 

 length. It is fastened at the lower end 

 by means of shellac to an aluminum wire, 

 G, which bears the mirror, the disk, and 

 the sphere, H. The wire is about 12 cm. 

 long and 0.075 cm. in diameter. The 

 sphere, as finally used, is of glass 1.499 

 cm. in diameter, the variation in its di- 

 ameters being less than 0.001 cm. It 

 weighs 4.495 grams. The disk finally 

 used is of glass. It is 1.339 cm. in diam- 

 eter, 0.3G4 cm. in thickness, and weighs 

 1.297 grams. It was made with as great 

 accuracy as the sphere. The sphere and 

 ^ disk are shown at Figure 2. The heavy 

 disk is made of lead. It is of the same 

 size as the glass disk, and weighs 5.793 

 grams. The mean radius of the lead disk 

 differs only by about 0.001 cm. from that of the glass disk. The differ- 

 ence in thickness is less than 0.001 cm. The disk is fastened on the 



B 



Figure 1. 



