The most expeditious method of determining altitudes ... with the ... barometer ; together with a description of the instrument, examples of its use ... To which is added a short description and use of T. Jones's patent sectograph / [Sir Henry Englefield].

Henry Englefield

Date:: 1811

Credit: The most expeditious method of determining altitudes ... with the ... barometer ; together with a description of the instrument, examples of its use ... To which is added a short description and use of T. Jones's patent sectograph / [Sir Henry Englefield]. Source: Wellcome Collection.

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$meter is three-tenths of an inch. This sized bore is fully sufficient to allow the free motion of the mercury. The cistern is of box-wood, turned truly cylindrical, and is one inch in its internal diameter, and an inch in depth *, a short stem projects from its top (the instrument being in a po- sition for making an observation), for the purpose of giving a firmer hold to the tube : this stem is perforated with an hole sufficiently large to admit the tube, which is glued to it in the usual mode. This tube projects into the cistern exactly to half its depth. I he bottom of the cistern is closed by a strong lid of box, which screws on the cistern, and pressing against a leather glued to the inside of the lid, renders the whole perfectly impervious to the mercury in every position. The tube being filled and boiled in the common way, and the instrument held inverted in a per- pendicular position, mercury is poured into the cistern till it is filled within two-tenths of an inch of the top. The lid is then firmly screwed on, and secured from being opened by idle curiosity, by a small screw passing through its side. The essential part of the instrument is now finished. The end of the tube in the cistern can never be uncovered by the mercury in any possible position, and of course no air can ever enter into it ; and, as the areas of the cistern and tube are as the squares of the diameters, the diameter of the bore of the tube being ,1, its external diameter ,3, and the diameter of the cistern 1,0, the area of the cistern is 100 — 9 Q\, and there being two-tenths of an inch left empty in the cistern, the mercury must fall 182-tenths, or 18 inches and two tenths, before the cis- tern is quite full; a space adequate to the measure of greater heights than any known mountain on the earth, much more so to any height in this country. It will not easily be believed, by those who have not seen it, that the air will act on a cistern thus completely closed, and of whicfr the wood, in its thinnest part, is above a quarter of an inch in thickness ; but the fact is, that even when the pores of the box wood are dosed by thick varnish, (except in that part which touches the mahogany tube), in order to prevent the wood from being affected by damp, the mercury-, on turning up the Barometer, takes its level almost instantaneously, certainly in less than half a mi- nute ; and that, when the instrument is suspended by the ®ide of the best Mountain barometer, of Ramsden’s con- sti action, with an open cistern, no difference w hatever can be perceived kr their sensibility to the variations of the atmosphere.$