ON THE MEASUREMENT OF TEMPERATURE. 
Headings of the barometer were made by means of two telescopes placed on a 
stand op230site the instrument. The barometer itself was 
attached to a pillar so that a card placed behind (fig. 3) 
it at an angle of 45° to the line of sight 
wave a 
O 
good 
illumination from a window at the side. The upper half 
of the card was black, the lower half white, and before 
taking a reading the card was adjusted so that the 
dividing line appeared about 1 millim. above the surface 
of the mercury. By this arrangement the meniscus 
ajjpeared perfectly sharp, as all surface reflections were 
completely eliminated. 
Mercury Thermometers .—The mercury thermometers em¬ 
ployed in these observations were compared with the 
thermometer em^jloyed to measure the temperature of the 
dead-space. This thermometer was itself compared with 
the 
gas 
thermometer and the error over the small 
range of temperature required, viz., 8° to 12° C., Avas 
known to within U'02°. The Aveiu’lit of such an error Avill be discussed later. 
G. The Constant-Volume Thermometer. 
'The apparatus employed in the determination of the pressure coeflicients of llic 
gases differs in many respects from that employed by other observers. Ciiaitujs 
enqdoyed thermometers of hard glass and of platino-iridium, and Oxnes ljulbs of 
hard glass; in both cases the connection between the thermometer bullj and the 
dead-space was made by means of a steel tulje connected with the glass Ijy means 
of cement. 
Beyond the fact that hard glass has a slightly lower coefficient of expansion than 
soda glass, there is no advantage in employing it; on the other hand, soda glass is 
more easily worked, and as by using it the bulb, stem, and dead-space can all Ije 
made in one piece, considerable advantage is to be gained. The bulb of our instru¬ 
ment (fig. 4) was made by sealing a piece of glass tube 30 millims. in diameter to a 
capillary tube 0‘75 millim. in internal diameter, Avhich for convenience in calibration 
Avas divided in millimetres for a portion of its lengtli. The stem Avas bent at right 
angles at C (fig. 4), and ultimately sealed at D to the portion of the stem leading to 
the dead-space. 
