Madras Journal of Literature and Science/Series 1/Volume 6/Further Remarks regarding a Plan for an Indestructable Barometer and Self-Registering Apparatus

by George Underwood

VI.—Further Remarks regarding a Plan for an Indestructable Barometer.—By Captain George Underwood, Madras Engineers.

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 of an indestructable and self-registering barometer, as iron was likewise mentioned by myself, I trust I may be pardoned for throwing out the following suggestions, although I am not certain that Mr. Taylor commented at all on the plan proposed by me, since "No. 2" merely is mentioned, without names being specified. The approval which that scientific gentleman bestows on the self-registering apparatus of No. 2, (deeming it very well worthy of attention and likely to be adopted, if the difficulties before stated in the syphon tube can be obviated), as it works on the same principle as that proposed by myself, induces me to hope that something may be yet done to bring the matter to a successful issue. For observatories and fixed stations a syphon tube of glass, or semi-transparent porcelain (as suggested by a friend), will at once remove the objections justly stated by Mr. Taylor, and in my first letter this recommendation was accidentally omitted, for at that time my attention was more particularly turned to the rendering barometers, if possible, indestructable, and more useful to travellers. The drawing annexed, (Pl. 4. fig. 1.) will, I really think, serve to show that an iron syphon tube may be so constructed as to be easily filled and carried about without the chance of air getting either into the longer limb, or into any situation from which it will not be immediately expelled, when properly prepared for action—(vide figure and remarks). By tapping the barometer while horizontal before working it, the air bubbles will rise to the surface near the stop cock; for since the cistern was nearly full of mercury originally, no additional quantity of air can ever enter it, since the stop cock will be made mercury tight: the passage of air through it will be advantageous. There is room also for the mercury to expand in high temperatures, and after a journey, when the instrument is laid horizontally, trifling particles of air, which may adhere to the sides of the cistern, can be readily expelled by heating the screw end of the cistern after opening the stopper. It will be manifest, also, on inspection before placing the barometer vertically, whether much air has entered the instrument, for the mercury to allow of such occupation must necessarily have been forced through the stop cock or screw; but this seems an impossible case, while the air itself can be so much more easily pressed through those places.

From trials with an iron barometer in my possession, I think the long tube should be of one uniform bore throughout, and not larger at the vacuum end, as before recommended by me, unless made by the most careful and expert workman, since the difficulty of boring the latter, and the trouble of expelling air from it, become very great; while the scale also requires more than ordinary attention.

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Remarks on the Figure.—The long limb A, is independent of the other, and the short tube B, with its cistern, stop cock, &c. screws on it at C. By this arrangement the long limb can be easily filled and the mercury boiled therein. When the short cistern limb is screwed on, while the other lays horizontally, mercury can be poured into the cistern to the level of the dotted line, and, from the peculiar shape thereof, the cistern will be as full as requisite, and very little air can remain in it on closing the stop cock. By inspection it will appear evident, that, turn the instrument as you will, the end of the long limb will be always immersed in mercury, and the contained air forced against the sides of the cistern. To keep the air in the cistern always in one part clear of the long or vacuum tube, if suspended in a box large enough to allow of its swinging freely on the hooks DD, no error will arise. The cistern end should always be carried uppermost in travelling or moving.

E. Iron bracket to support and steady the short limb.

F. A suspending iron and ring at the end of the long limb.

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