Page:Madras Journal of Literature and Science, series 1, volume 6 (1837).djvu/87

1837] Now, presuming the instrument-maker has thoroughly expelled the air on first filling the instrument, all that the possessor would require to do in order to satisfy himself as to the pressure of air in any portion of the tube, would be to incline it in order to fill the long limb, turn the stop-cock k and screw home the cistern screw. If the mercury now stands at the height mentioned on the instrument, it of course must be as perfect as when it left the maker. The long limb ought to be sufficiently capacious to contain all the mercury in the glass limb, and part of that in the cistern, so that when the stop-cock above the latter is closed, there will be as much air in the cistern as will allow space for the expansion of the quicksilver in the whole tube, while this air is prevented entering the long limb, as the end of this will always be immersed in mercury. A simpler form than the above would consist in adopting that of the Englefield barometer, with the addition of a short glass limb rising out of the cistern, the long limb having expanded top, to contain the mercury in the short one, and part of that in the cistern as above mentioned, if this arrangement were applied to M. Gay Lussac's barometer, the possibility of air entering the long limb would be much more likely to be prevented than by the inverted cone, which after all, does not prevent the entrance of air, but merely confines what may have got into the lower part of the mercurial column. The instrument also might be carried in any position, which at present it does not admit of without the danger of air entering.





In the remarks of the Astronomer of Madras, respecting the Indestructable Barometer and Self-Registering Apparatus, recommended by Dr. Gilchrist, I observe that Mr. Taylor admits that an iron tube can be rendered air-tight by the application of caoutchouc varnish, but adds that this precaution was eventually of no avail on the application of fire, for the purpose of boiling the quicksilver in the tube. Now, since the mercury may be boiled in the tube before applying the varnish, I cannot understand why the iron should be condemned for the reason alluded to. True it is, that, with such a material, air bubbles cannot be discerned, and with any opaque substance the liability to error is vastly greater on that account, if some means cannot be adopted for ascertaining the existence of so much air as shall vitiate the observations. However, in furtherance of some hasty and brief recommendations which I ventured to forward to the Journal on the subject