THE MEASUREMENT OF TE.MPERATUEE. 
121 
AVlien the teoiperature of the bulb and column were steady, the mercury in the 
dead-space was adjusted first of all by means of the mercury reservoir and sto^j-cock 
n, and finally by means of the pinch-cock m. The mercury was always brought to 
within about OT millim. of the j^oint. After tapping the support with the hand, 
and waitinor a short time for the mercury to settle down, the observations were made. 
The spider-line of the micrometer was first brought into coincidence with the line 
on the scales next above the meniscus, and then with the top of tlie meniscus itself, 
readings being taken in both positions. A light was then held in front of the 
instrument, and the height of the meniscus was determined by bringing the spider 
line into coincidence with the point at which the vertical bright line, caused by the 
reflection of the ligdit from the curved surface of the mercury column, came to an 
end. The height of the mercury meniscus was applied to the calculation of the 
correction to he applied for capillarity (Kohleausch). The determination of the 
position of the point e with regard to the scale was also necessary for the calculation 
of the distance of the mercury from it, and from this the volume of the dead-space. 
Its value, and the value of one division of the micrometer screAv in terms of a 
division of the scale, were measured separately. 
Observations of the pressure were invariably made by each one of us alternately, 
the other reading the thermometer enclosed in the water jacket, and the thermometer 
placed close to the hoi'izontal portion of the stem. The position of the lower 
meniscus Avas obserA-ed, then that of the ujjper meniscus, and finally the l(3Aver 
meniscus again. The level of the mercury in the a})})aratus aaes usually readjusted 
hetAveen each pair of measurements. 
IG. The Ice-Point. 
The bulb ANais immersed to Avithin 40 millims. of the horizontal })oition of the stem 
in an inverted glass bell-jar, and surrounded Avith broken ice packed tightly round it. 
The bell-jar A\-as filled nearly to the top of the A’essel A\dth distilled AA ater. fSamples 
of the ice Avere on two occasions melted and the liquid evaporated ; in neither case 
Avas it found to contain more than a minute trace of solid matter. A piece of Avood, 
AA’ith a slit cut in it for the stem of the thermometer, aa’us placed on top of the bell- 
jar containing the ice. 
17. 2'lie Boiling-Poiut. 
The bulb of the thermometer Avas surrounded Avith dry steam by means of a large 
double-AA'alled copper hyjrsometer of the usual form. The stem of the thei'inometer 
passed through a hole in the cover of the hypsometer, and the aperture A\as made 
fairly steam-tight by means of a small strip of Avet rag Avound round the stem. No 
correction of the diflerence of })ressure inside and outside the hypst»meter aavis 
necessary. 
VOL. cc.—A. i; 
