A. L. Day and J. K. Clement — Gas Thermometer. 415 



accessible for arranging an ice bath for the zero reading. 

 Hydraulic power served to raise and lower the furnace con- 

 veniently. "When the furnace was raised for heating, a circle 

 of bolts provided a positive pressure upon the top joint. 



The Manometer. — The manometer was located about l/2 m 

 distant from the furnace and was of the usual U-tube type, 

 constructed with a very heavy cast-iron base and light upper 

 parts in order to render the mercury columns as free as pos- 

 sible from the vibrations of the building. The fixed point to 

 which the mercury level was always adjusted occupied the 

 usual position at the top of the short arm, the other arm 

 extending upward for a distance of about two meters. 



The scale, which was 1*8 meters long, was immediately 

 beside the long tube, and was provided with a sliding vernier 

 reading to O01 mm . It was of brass with a silver-plated band 

 upon which the divisions were ruled and had been calibrated by 

 the German Normal Aichungskommission in Charlottenburg. 

 The length of any portion of it was known in terms of the 

 German standard meter to the nearest '01 inm . The scale was 

 fixed in position below and arranged so as to expand upward 

 through appropriate guides against a rubber cushion with the 

 changes in the room temperature. The long manometer 

 tube also passed through three guide screws at the top of 

 the apparatus, which allowed it to expand and contract 

 unhindered. Headings were obtained by means of two paral- 

 lel knife edges on the vernier carriage, which could be brought 

 to accurate tangency with the mercury meniscus by a slow 

 motion screw provided for the purpose. The mechanical con- 

 struction was extraordinarily rigid and very satisfactory. The 

 temperature of the scale and mercury columns was obtained 

 from three thermometers, each set in a short tube of mercury 

 after the manner of Holborn and Day.* Each tube with its 

 thermometer could be moved up and down close beside the 

 scale and mercury columns so as to give the temperature of 

 the top, middle and bottom of the longest column. The 

 observed temperature differences along the mercury column 

 sometimes amounted to 1°. This would not affect the scale 

 length by a dangerous amount, but the average temperature of 

 the mercury column requires to be known to about 0*2°, 

 with the high sensitiveness of this instrument, in order to 

 bring the errors in the pressure determination within the 

 desired limits — hence the three thermometers. 



The mercury supply was contained in two basins, one a 

 hollow steel bomb enclosed within the cast-iron base of the 

 instrument, and the other a steel flask mounted upon the wall 



* Holborn and Day, On the Gas Thermometer at High Temperatures, this 

 Journal (4), viii, p. 170, 1899. 



