98 Report on the Instruments employed in, and on the 



in the level of the mercury in the latter; thus the neutral 

 point, i. e. the level from which the height of the mercurial 

 column was first measured being known, the correction for 

 capacities is readily made, by subtracting or adding one 

 hundredth of the difference, accordingly as the mercury has 

 risen above or fallen below this point. The tube is armed at 

 the bottom with a platinum cap perforated with a hole of one 

 eighth of an inch in diameter, which is sufficiently large to 

 admit of the mercury flowing in and out with perfect freedom. 

 This platinum guard has been well wetted with mercury, 

 according to the suggestion of Mr. Daniell, in the 39th 

 Number* of the Journal of the Royal Institution, and is in- 

 tended to prevent, by the complete contact which it insures, 

 the gradual insinuation of air and moisture, which there is 

 reason to suppose takes place in all barometers, from the 

 want of affinity between the glass and the metal. 



The mercury was very carefully boiled in the tube ; but 

 notwithstanding the utmost attention was bestowed upon this 

 very difficult process, some small air-bubbles made their 

 appearance at the top of the column, immediately after the 

 instrument was fixed in its place. Few people can be aware, 

 except those who have tried the experiment, of the difficulty 

 of boiling in a glass tube such a weight of mercury as that 

 contained in this barometer; a difficulty which will always 

 render it advisable to have recourse to tubes of small diameter, 

 except in standard instruments, in which it is desirable to 

 reduce the capillary action to the smallest possible amount. 

 This correction does not exceed 0.009 inch in the present 

 instance, but is constantly applied to the observations. 



* Journal of Science, &c. for October, 1825, Vol. xx. page 88. 



