84 Mr, S. W. Holman on the Relation between 



and p 3 by reading three mercury columns. Also we can obtain 



TZ 4 \ 

 a value of l 2 from readings of the gauges when S 1 = 8 2 , 



R^ 

 which needs only to be corrected for expansion of the glass 

 to be used directly in equation (5). The whole process is thus 

 reduced to the simple matter of reading columns of mercury, 

 no measurements of volumes of gas being necessary. The 

 nature of the correction of R and X for temperature appears 

 by putting into the above formulas, in which these values are 

 supposed to be for 0° C, the coefficients of expansion of the 

 glass = A ; we thus get from (5), 



%_EJ(i+A8 1 )%(l+AS 2 ) p\-p\ l + «8 2 



% BJ(1 + AS,)%(1 + A8,) P l-Pl l + «Si 

 _ Bj(l+A8Q% Pl-pt i + aS 2 

 R<(1 + AS 2 )% ' pl-pl ' 1 + aSl' ■ 



(6) 



Lest, however, an error might occur in the last reduction 

 from a difference between the coefficient of expansion of the 

 bore of a capillary tube and of its lineal expansion, I have care- 

 fully measured both, and find that the coefficient for the bore 

 is 0-0000075, while for the linear expansion I find 0*0000080 

 per degree Centigrade, a difference too slight to affect the re- 

 sults in my use of it; I have thought it best to use the value 

 0*0000075, as it entered in the fourth power, while the other 

 entered only in the first power. The tubes used have also been 

 calibrated to ensure the selection of those of uniform bore ; and 

 their dimensions have been accurately measured by mercury 

 and a micrometer-screw. The dimensions of the two tubes 

 used in the experiments to be described were, for tube No. I., 

 X=1272-3 millims., R = 0'1098 millim. ; for tube No. II., 

 X= 1274-1 millims., R= 0-1115 millim. 



To make an experiment with this apparatus, it is merely 

 necessary to start the jet of water and allow the exhaustion to 

 proceed until the mercury columns in F and E have come 

 completely to rest. Readings are then taken of the heights of 

 these columns, by means of a cathetometer, from a steel scale 

 placed beside the gauges. The reading of the barometer cor- 

 rected for instrumental error gives the pressure at A. All 

 these are reduced to the freezing-point ; and E and F are cor- 

 rected for capillarity by the Tables of Delcros. The tempera- 

 ture of the baths is also taken by thermometers in various po- 

 sitions in the troughs. This must be kept constant throughout 

 the experiment ; and I have therefore principally used the tem- 

 peratures of melting ice and boiling water. In the experi- 



