ACOUSTICS AND GRAVITATION. 



139 



105. Observations. The beginning and end of a series are indicated by 

 circles. These were made on the same plan as above, readings, y, being taken 

 at intervals about 30 minutes apart, and they are given in figure 176. The 

 double amplitudes, Ay, are shown in figure 177. The circles distinguish a. m. 

 and p.m. means. The observations in a steam-heated room with its variable 

 temperature (20 to 30) would be inadmissible; but here, where a test is aimed 

 at, this change of temperature is even desirable. At the outset a few observa- 

 tions were taken for a plenum of air. These have the usual excessive value, 

 being on the average Ay = 2 . i cm. for the morning and Ay = i .6 for the after- 

 noon periods. It should be Ay = 0.38 cm. The smaller values of Ay occur 

 when the steam heat is shut off. 



09 



2 {V 



177 



= o u -o' 



1 Itacuwn-l 



Oct. 



The observations for the needle in vacua (fig. 176) lie indifferent regions, 

 for the reason that a number of adjustments had to be made at the distant 

 telescope and scale. These moved from day to day as the temperature of the 

 room varied, without, however, affecting the differences or double amplitudes 

 Ay. The latter (fig. 177) have a definitely larger value as a whole than in the 

 preceding paragraph, ranging here from Ay = 0.3 7 cm. (rare) to Ay = 0.55, if 

 we discard the exceptional value (0.66) obtained with a open window. The 

 mean values in the morning were Ay = 0.49 cm. and in the afternoon Ay= 

 0.47 cm. Both are much too large (nearly 30 per cent) and the curve of suc- 

 cessive values of Ay is inadmissibly meandering. The mean summer data of 

 the preceding paragraph for the same apparatus (IV) and location were Ay= 

 0.44 cm. in the morning and Ay = 0.43 cm. in the afternoon. The effect of a 

 heated room is thus an increment from Ay = 0.43 to Ay = 0.48. Moreover, 

 in the latter case there is no equally appreciable correction for stem attraction 

 as in the former. Vacua from 0.6 cm. to 6 cm. were employed, but no difference 

 was detected attributable to this cause. Obviously, the radiant forces now 

 impinge on the shot, the thin stem of the needle having no advantage over the 

 preceding cases. 



106. The residual radiant forces. It is interesting to inquire as to the 

 amount of pressure variation and velocity of air-currents implied in the esti- 

 mate for radiant forces. The energy equation may be written, if (p Ap) = p 

 nearly, 



