ACOUSTICS AND GRAVITATION. 



121 



the location of the small leak in the apparatus, which after several complete 

 overhaulings of the apparatus had escaped detection. The chief purpose, how- 

 ever, was that of surrounding the needle with a medium of large heat-content 

 and slow variation of temperature. 



The arrangement is shown in figures 154 (plan) and 155 (elevation), where 

 mm is the needle with its mirror n, quartz fiber q suspended from a sealed 

 torsion-head at the top of the tube t. The needle is surrounded by the case 

 ww of waxed (impregnated) wood, closed on both vertical sides by plate-glass 

 windows. The case is supported by the posts rr held in clutches anchored in 

 the pier. The tube for exhaustion is at s. The water-bath BB surrounds the 

 case completely, and the parts rrs pass 

 through gasketed devices to prevent leak- 154 



age. The rod v, also gasketed and clutched <73 

 by an arm fastened in the pier, carries the c 

 sleeve u within the water-bath. It is to 

 this that the crank-like arm carrying the 

 weight M is attached and free to revolve 

 from M to M,' the arc being limited by 

 appropriate stops. 



The circular trough of the water-bath 

 was about 30 cm. in diameter and 20 cm. 

 high; the rectangular part 25 cm. long 

 and 14 cm. broad. It contained over 30 pounds of water. The lead weight M, 

 whose effective density is now 1 1 .3-1 , was approachable to i or 2 mm. of the 

 glass walls of the case, the distance between centers M, m being left unchanged. 

 The case was leveled and the tube t supported by separate clutches, as above. 

 Moreover, to keep the glass window z parallel to the case, it also was secured 

 by a special clamp from the pier. 



The results obtained with this complicated apparatus were not at first as 

 orderly as expected from so large a mass of water, nor was the leak of about 

 5 cm. per hour on complete exhaustion removed. The conformity to the tem- 

 perature of the water-bath was extremely slow. At first, at the high vacua, 

 p = 4 to 8 cm., the results seemed to give promise; the readings were ^ = 6.95, 

 7.50, 7.00, 7.60, 6.95, 7.60, etc., in five-minute periods. Later, however, at a 

 slight exhaustion, p = 68 cm., the mean excursions in successive hours were 

 Ay = 0.69, 0.93, 1.02, etc., the normal excursion in the absence of the water- 

 bath being over twice as large. It therefore seemed preferable to begin the 

 work with long-period observation (per hour) for some time, in a manner simi- 

 lar to the case of figures 119, 120 of apparatus I. 



Next day, at = 67 cm., the results as shown in figure 156 a conformed 

 to Ay=i.i6; but they are otherwise no improvement on the prior results. 

 Thereafter the apparatus developed a leak and had to be taken down to be 

 thoroughly overhauled. It was then replaced and observations were made 

 under full air-pressure. The apparatus having had a day for cooling, the 

 excursion in the absence of water in the bath was again normal in behavior, 



