CHAPTER X. 



GRAVITATIONAL EXPERIMENTS, CHIEFLY WITH REFERENCE TO THE 

 ACCOMPANYING RADIANT FORCES. 



82. Introductory. In the course of the work of the last report, I came to the 

 conclusion that it would be worth while to study the motion of the gravitating 

 needle subjected to alternating forces, in air, before beginning the more dif- 

 ficult experiments in vacuo, which follow. The following paragraphs and 

 figures contain examples of investigations made with this end in view. These 

 results are very interesting, even if considered as evidence of the pervasive 

 character of thermal radiation. The profound effect produced by thermal va- 

 riations scarcely exceeding a degree per day is astonishing. One is tempted 

 to ask whether some other type of radiation is not intermixed with the ther- 

 mal effect.* 



83. Apparatus. The apparatus was not materially changed from that here- 

 tofore used. The needle consisted of two lead shots, each weighing 0.59 gram, 

 held at the ends of a shaft of straw 22 cm. long, the latter being retained in 

 favor of the radiations in question. The latter with appurtenances (mirror, 

 etc.) weighed 0.58 gram, about the same as either shot, while the straw shaft 

 alone weighed but 0.28 gram. The whole was supported by a quartz fiber 40 

 cm. long, attached to a torsion-head above, and inclosed by a glass tube as 

 far as the needle. 



The quartz fiber and glass tube and the needle were additionally protected 

 by a flat case made up of two parallel plates of glass, separated on each vertical 

 side by a strip of wood 1.5 cm. thick. These wooden strips projected beyond 

 the glass plates above and were provided with two cylindrical iron holders (gas- 

 pipe nipples) by which they were attached with clamps tc the pier, independ- 

 ently. The waste space above the needle was filled with sheets of cotton 

 batting. The bottom of the case was closed with a similar strip of wood 

 covered with cloth and was held in place by friction. 



The attracting weight, M = p4p grams, moved on a crank supported by 

 separate attachments anchored in the same pier. Stops were placed so that 

 M could be quickly rotated from one side of the shot to the other, the distance 

 between centers being on either side .R = 4. 24 cm. 



The deflections of the needle were read off by a mirror and scale method, 

 taking advantage of the half-silver plate device, described in the last report. 

 As the telescope is at half the usual distance, the magnification is doubled. 

 The centimeter scale, illuminated by a horizontal tungsten lamp behind it, 

 was at a distance of 378 cm. from the mirror on the gravitating needle. 

 Hence the angles are 



*Mr. Boys has recently given an interesting review of this subject in Nature, p. 40, 

 Sept. 8, 192 1. 



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