ACOUSTICS AND GRAVITATION. 



133 



As the external weight, M = g^g grams, was the same as before, and the dis- 

 tances between centers 4.2 cm., as well as length of needle also, the double 

 deflections Ay to be anticipated in comparison with the former apparatus 

 would depend merely on the scale-distances from the mirror. This ratio was, 

 with all allowances, about 0.7, so that the new deflections should not have 

 been much smaller than the old. They were, however, 5 to 10 times smaller, 

 showing that with the new needle and new location, radiation forces of enor- 

 mous amount (relatively) had been eliminated. 



The first experiments were made in a plenum of air. They are given in 

 figure 1 66, the scale-readings of the chart (y) being taken at regular intervals 

 about half an hour apart, both in the morning and afternoon. Even if we 

 discard the readings on October 17, when the sun accidentally entered the room, 

 the deflections are far from regular and there is considerable drift. Table 6 

 shows the mean double deflections (scale-readings) Ay in centimeters (scale- 

 distance 261 cm.) for the mornings and afternoons of successive days. The 



values are least on dark days and large on clear, sunny days, as in the work 

 above. In spite of the filamentary shaft of the needle, therefore, the data are 

 still quite unsatisfactory, ranging from 0.70 cm. to 1.5 cm., and in the morning 

 they are usually larger. 



The apparatus was now exhausted to about i cm. of mercury, and the experi- 

 ment repeated in the same way. The new data (scale-readings y) are given in 

 fig. 167, p. 138. The results are not only smaller in amplitude , but much more 

 regular, showing that much of the radiation effect has been eliminated. Never- 

 theless, there is some drift in the lapse of time, to be attributed to radiation- 

 pressure. The double amplitudes Ay in table 6 , constructed graphically in 

 fig . 1 68 (p . 1 3 7 , on a tenfold larger scale) , bear out the same inference. Ay varies 



