PNEUMATICS. 



*mr 



mercury, the tube being 

 placed in a truly vertical 

 position, the altitude then 

 denoted by the scale C F 

 will be the height of the 

 barometer. 



To the scale C F a ver- 

 nier or nonius may be at- 

 tached, to give greater ac- 

 curacy to the observation, 

 by which the divisions can 

 be read to the one-hun- 

 dredth part of an inch. 



(19.) In order to com- 

 pare barometric observa- 

 tions made at different 

 times or places, it is neces- 

 sary to observe the tem- 

 perature of the mercury 

 which composes the baro- 

 metric column. For mer- 

 cury, like all other bodies, 

 is dilated by heat ; so that 

 at different temperatures 

 the same weight of mer- 

 cury stands at different 

 heights in the tube. Tables are accord- 

 ingly constructed, and rules given, by 

 which the heights of the mercury in the 

 barometer may be reduced to what they 

 would be, if* the temperature of the 

 mercury had been fixed, and the same 

 as that of melting ice. 



(20.) The entire play of the mercury 

 in the barometer not exceeding three 

 inches, minute variations in the weight 

 of the atmosphere will produce so small 

 a change in the altitude of the column, 

 that the observation of it is attended 

 with considerable difficulty. Various 

 contrivances have been suggested to in- 

 crease the range of the mercury in the 

 tube, and render small changes percep- 

 tible : one of the most obvious would be 

 to use a lighter fluid, instead of mer- 

 cury ; but to this there are various prac- 

 tical objections. The following con- 

 trivances for enlarging the scale may be 

 mentioned : 



(21.) The diagonal barometer is one 

 in which the tube AB (fig. 7.) is bent 

 from the vertical position, at a point C 

 less than twenty-eight inches from the 

 level A of the mercury in the cistern ; 

 and the inflected part of such a length, 

 that the perpendicular altitude of its 

 extremity' B above A, shall exceed 31 

 inches. *To determine the relation be- 

 tween the scale of such a barometer and 

 the common vertical one, let the line A 

 C be continued upwards from C, until 

 it meet the horizontal line B D, drawn 



through B. Let the 

 scale C D be then 

 made as for the 

 vertical barometer, 

 and parallels to 

 BD, from its points 

 of division, will give 

 the scale for C B. 

 It is evident that 

 the divisions will 

 thus be enlarged, 

 in proportion as the 

 tube C B is deflect- 

 ed from the vertical 

 position C D. 



(22.) Another 

 contrivance for in- 

 creasing the scale, 

 is the tcheel-baro- 

 meter. This instru- 

 ment consists of a 

 bent tube ABC 

 8.) closed at 



/<7-7. 



A, and so that 



(Jig. 



the distance C A shall not be less than 

 about thirty-one or thirty-two inches : 

 the end C being open, the tube is filled 

 with mercury. The mercury wall sub- 

 side in the leg A B until the difference 

 of the levels E F will be 

 equal to th-3 height of a 

 column of quicksilver 

 which balances the pres- 

 sure of the atmosphere, 

 and every change in the 

 level of E will be accompa- 

 nied by an equal change, 

 but in the opposite direc- 

 tion, in the surface F; so 

 that the change in the 

 height of the barometric 

 column, is double the 

 change of the level F. On 

 the level F there floats a 

 small iron ball, to which 

 a string is attached, 

 which is passed over a 

 pulley P, and to which a 

 weight W less than that 

 of the ball P is suspended. 

 The axis of the pulley P 

 passes through the centre 

 of a large graduated cir- 

 cular plate G, and carries an hand or 

 index H, which revolves when the pulley 

 is turned. 



In this apparatus, when the mercury E 

 rises, and F falls, the floating ball, not 

 being completely balanced by "W, falls 

 with it. The string being pressed bv 

 the weights on the wheel P, turns it, 

 and with it the index which plays on the 

 graduated circular plate. The contrary 



