90 



Mr. S. A. Sworn. 



difference for any of the instruments. By comparing the observations 

 of the boiling point made with the vertical and horizontal ther- 

 mometers, the coefficient of internal pressure may be determined, as in 

 the vertical position the hydrostatic pressure of the mercury acts on the 

 bulb. The internal pressure correction (/?;) is connected with the 

 external pressure correction (/?e) by the relation 



where k is the compressibility of glass. The following table gives the 

 comparison between the observed and calculated value of Pi; in 

 degrees per millimetre pressure of mercury. The column headed li 

 shows the length of the mercury column between the boiling point 

 and the centre of the bulb. 



Thermometer. 



(calculated). 



Pi (observed). 



h. 



K (assumed). 



4976 



-0001229 



-0001241 



63-8 



0-0000154 



711179 



-001159 



0-001172 



57-3 



-0000127 



2218 



000785 



-000804 



56-7 



-0000143 



2219 



-000825 



-000878 



59 -2 



-0000143 



The observed values of fii were obtained by dividing the differences 

 in the observed reading (horizontal — vertical position) by the height of 

 the mercury column above the centre of the bulb. No correction was 

 made for the fact that when the column of mercury was raised from 

 the horizontal to the vertical position, the thread descended, and the 

 reading therefore corresponded to one taken with a falling temperature, 

 while in the horizontal position the reading corresponded to one taken 

 with rising temperatiu*e. Mr. Sworn concluded from the good agree- 

 ment between the observed and calculated values of Pi that the effect 

 of capillarity is somehow eliminated in these observations. 



Direct observations were made on this point. If the hypsometer is 

 observed at an angle to the horizontal, the readings should differ by 

 the quantity K according as the thermometer is brought into its 

 position from the horizontal, or from the vertical. The observed 

 differences are, however, for the most part nil^ and in any case a mere 

 fraction of K. If, further, the thermometer is gradually raised from the 

 horizontal position, the observed differences in the readings should be 

 expressible in the form h^i sin ^ - K, where h is the total height of the 

 mercury column, but in reality they are well expressed by leaving K 

 out of account. The author remarked on this point : — 



" The effect of capillarity on the advancing or receding columns is 

 unquestionably liable to compensation, either by vibration or by 



