45, COENHILL, E.G., AND 122, EEGENT STEEET, W., LONDON. 



15 



FIG. 18*. 



22. Negretti and Zambra's Howson's Patent Long 

 Range Barometer. 



" The object of this instrument is to add to the sensitive- 

 ness of the ordinary mercurial column, by giving it an increased 

 range, a desideratum which it appears to accomplish with 

 simplicity and efficiency. 



" The principle of construction -will be understood on refer- 

 ence to the diagram, fig. 18, which represents a section of the 

 working parts of the barometer divested of its case. 



" A is the barometer tube, which is of large dimensions, 

 and of greater length than usual in proportion to the additional 

 length of range which it is intended to apply to it. The cistern, 

 B, is of a tubular shape, so as to contain a fixed depth of 

 mercury, also determinable by the range. To the bottom of 

 this cistern is attached, concentrically, a light glass stem or 

 long hollow tube, S, hermetically sealed, springing to a height 

 of about 28 inches above the fixed level of the mercury in the 

 cistern. 



" When all the parts are in situ, as in the diagram, fig 18*, 

 the tube A being freely suspended, and the whole filled with the 

 requisite quantity of mercury, the immediate result of the arrange- 

 ment is that the cistern hangs in suspension without any fixed 

 support. The stem C, it will be observed, passes up the tube 

 A, and terminates a little below the upper level of the mercury 

 M : its upper end is therefore exposed to no more downward 

 pressure than that caused by the weight of the mercury above 

 it, and consequently there is an excess of upward pressure from 

 the atmosphere exteriorly which tends to raise the cistern. 



" If we suppose, for instance, the area of the stem to be half 

 a square inch, and its top to be covered with 1 inch in depth of 

 mercury (the space above being of course a vacuum), there will 

 be a pressure tending to push the cistern downwards of only 

 J Ib. or thereabouts, while the atmosphere will be pressing 

 upwards on an equal area with a force of 7 Ibs. or more. Thus 

 it will be seen that when the excess of upward pressure is 

 exactly balanced by the weight of the cistern with its stem, 

 and contained mercury up to the level &, an equilibrium will 

 be established which will keep the cistern stationary. If from 

 any cause the cistern should become lighter, it will ascend : if 

 it should become heavier, it will descend, and the extent to 

 which it will move in either case will be limited by the immer- 



