120 PROCEEDINGS OF THE AMERICAN ACADEMY. 



and over the middle of the bath support, with which, however, it was not 

 allowed to come in contact. To this bar were firmly wired and cemented 

 with marine glue the tube from the top of the bulb aud the upper end of 

 the side tube. Thus the fixed position of the bulb was secured by two 

 supports independent of the building and of each other, — at the bottom 

 by the frame of the bath, and at the top by connection with the iron rod. 

 Both supports were unaffected by temperature changes, because of con- 

 stancy in the temperature of the room. The frame of the Lord Rayleigh 

 barometer was likewise indej:>endently bolted into position. 



The top of the wooden support around the bath was completely covered 

 with a large plate of glass, through a hole in the middle of which rose the 

 upper edge of the bath itself. The joint between the two was filled with 

 marine glue. The cover of the bath was of felt incased in black enamel 

 cloth. By having everything 'at the mouth of the bath thus capable of 

 being washed, the chance for the introduction of impurities was reduced to 

 a minimum. 



A bulb made as nearly as possible like the one used in the preliminary 

 experiments was again employed, and no essential change was made in 

 its arrangement. The lower end of the bulb tube, however, to which 

 the rubber tube leading to the mercury levelling reservoir V had formerly 

 been directly attached, was made to dip into a cup completely filled with 

 mercury and closed at the top by a rubber stopper cemented over with 

 marine glue. This cup, not shown in the diagram, was provided with a 

 side tube near the top to which the rubber tube was attached. This 

 arrangement served as a trap to catch any possible sulphide of mercury 

 formed by the action of the mercury and the rubber and to prevent its 

 passage into the bulb. 



In the Lord Rayleigh barometer, which determined the constancy of 

 the pressure throughout the experiment, the position of coincidence of 

 the mercury surface with the lower point (1) of the iron rod (F) was 

 now found by the making of electrical contact instead of by observation 

 through a small lens as before. This electrical method was made 

 possible by cutting the iron rod in two parts and by slipping a very thin 

 sheet of mica for insulation between them. The parts were firmly 

 fastened together again by a collar made of a closely fitting glass tube 

 held in sealing-wax. This change is not indicated in the diagram. The 

 lower end of the iron rod was provided with a sharp platinum tip, which 

 allowed greater accuracy in setting. 



In order to read accurately the position of the upper meniscus with re- 

 spect to the upper iron point, a background, black above and white 



