168 Mr, Nixon on the Theory of the ■ [Sept. 



hermetically closed at one end, filled with mercury,* and having 

 the other immersed in a cylindrical ivory or w^ooden cistern B, 

 containing a sufficient quantity of the same fluid. The tube and 

 cistern are connected by the cover C, having a small aperture 

 for the admission of air. 



To render the instrument portable, the cover is constructed 

 without the aperture, but can be opened for the ingress of the 

 air, or closed to prevent the escajie of the mercury by means of 

 an ivoiy screw. To counteract the oscillations of the mercury 

 within the tube, the central part of the bottom of the cistern is 

 formed of leather having a screw fixed beneath it capable of 

 forcing the mercury to the summit of the tube. 



At whatever height of the mercury within the cistern the 

 artist may have adjusted the scale of inches so that its zero shall 

 coincide in level with the surface of the fluid, it is obvious, that 

 as the pressure increases (or diminishes), the mercury will sub- 

 side below (or rise above) the level of zero. This constant 

 Bource of error is remedied in the barometer of Ramsden by rais- 

 ing or depressing the leathern bag sustaining the mercury until 

 the mark made on a piece of ivory floating in the cistern coin- 

 cides in level with some fixed point equally elevated with the 

 mark above the zero of the scale. The adjustment of the float 

 to the gauge being considered as difficult, a superior plan has 

 been adopted by Troughton, consisting in having the vertical 

 sides of the cistern formed of glass cased in brass. Near the 

 top of the case is a horizontal sht, and another exactly opposite, 

 both having their upper edges accurately of the height of the 

 zero of the scale. The adjustment is eflected by turning the 

 screw of the leathern bag until the surface of the mercury seen 

 through the horizontal slits just excludes the transmitted light. 



Barometers of this description are extremely cumbrous 

 difficult. in ordinary hands to adjust, and objectionable on 

 account of the mercury being exposed during an observation to 

 humidity, &c. They have in consequence been superseded for 

 general use by the one invented by Sir H. Englefield. Its pecu- 

 liarities consist in the cover of the cistern being permanently 

 closed, the air forcing its way through the pores of the wood ; 

 and that instead of an adjustment to bring the surface of the 

 mercury constantly to the level of zero, the computer increases 

 or diminishes the observed heights of the columns, according as 

 they exceed or fall short of the height at which the scale was 

 adjusted (termed the neutral point) by a quantity found by divid- 

 ing those diflerences by the ratio of the area of the mercury 

 within the tube to its area within such part of the cistern as 



• When the barometer, filled with -mercury at ordinary pressures, is carried to the 

 summit of a lofty mountain, a third, or even one-half of the mercury within the tube 

 may subside, and mixing with that in the cistern will render the boiling of it within the 

 lub« of very limited utility. 



