BAROMETER. 



293 



Barometer. 



Syphon 

 brfometer. 



Inclined 

 barometer. 

 Plate 

 JL.III. 

 Fig. 8. 



Rectangu- 

 lar baro- 

 meter. 

 Plate 

 I.III. 

 Fig. 9. 



and exhibiting It on a circular dial plate. For this 

 purpose, a piece of ivory of a bell form is made to 

 float on the surface of the mercury in the shorter 

 leg, having a silk thread fastened to its upper end, 

 which, passing over a pulley, is stretched by a weight 

 that is nearly a counterpoise. By this means the mo- 

 tion of the mercury is communicated to an index, 

 which turns round a graduated circle, and thus the 

 vertical range is enlarged at pleasure. The motion 

 of the index in the wheel barometer has been render- 

 ed more sensible by Mr Russel of Falkirk, by the 

 addition of a little wheelwork. 



Under the same figure we may refer to the syphon 

 barometer ; for the form of the tube, and the motion 

 ef the mercury, are the same in both. This, at least 

 in theory, is one of the most elegant modifications of 

 the Torricellian tube. It consists of the tube appli- 

 ed to a frame of wood, with the addition of a sliding 

 bar of brass nearly of the same length. At the lower 

 end of the bar is an index, which, in observing the 

 mercury, is to be set opposite its surface in the short- 

 er leg : the upper end of the bar is divided into a 

 >;cale of inches, whose zero is the index below. The 

 scale is furnished with a sliding hand for pointing to 

 the surface of the mercury in the longer tube, and 

 also with a vernier for distinguishing the smaller divi- 

 sions. By this simple contrivance, the sum of the 

 motions of the mercury in both legs is correctly ex- 

 hibited, and measured at top. 



The inclined or diagonal barometer (Fig. 8.) 

 is another form of this instrument, for augmenting 

 the scale. It differs from the common barometer in 

 having the upper part of the tube, where the scale is 

 applied, bent at B, so that the range is on AB instead 

 of a B. By this arrangement, when the mercury 

 stands at d in the Torricellian tube, it will stand at D 

 in this instrument, D d being parallel to the horizon. 

 Hence the scale, compared with that of the common 

 barometer, will be enlarged in the ratio of A B to a B ; 

 a ratio which admiu of unlimited increase, since AB 

 may be made indefinitely great with respect to a B. 



Of all the methods which have been proposed for 

 increasing the range of the barometrical scale, this 

 seems to be the most susceptible of accuracy. The 

 only objections to which it is liable, are, the friction- 

 arising from the increased column of mercury against' 

 the sides of the tube, and the possibility of the mer- 

 cury separating into detached portions during its de- 

 scent in the inclined part. The invention of the in- 

 clined barometer is generally ascribed to Sir Samuel 

 Moreland ; but this is doubtful. Derham, who has 

 described it in the Phil. Trans, for 1698, No. 236, 

 only mentions that it had beeu communicated to him 

 by a friend. The invention itself is so obvious in 

 principle, that it scarcely merits enquiry to whom it 

 is due. 



The rectangular barometer (Fig. 9.) consists 

 of a tube ABC btnt at right angles, and swelled out 

 at AD, which includes the vertical range of the mer- 

 cury into a cylindrical cistern. The scale is reckon- 

 ed on the horizontal part of the tube CB, and may 

 be enlarged to an unlimited extent, by making the 

 bore of the tube indefinitely small, in comparison of 

 the cistern AD. The mercury is prevented from 

 flowing out at C, by the pressure of the air acting 



upon its surface at E. When the weight of the air 

 is increased, so 3 3 to produce an ascent of one inch of 

 the mercury in the Torricellian tube, it will also pro- 

 duce an ascent of one inch in AD ; but in order to 

 do this, the mercury must be supplied from CB, and 

 the space through which it moves from E towards 

 B, will be to one inch, as the square of the diameter 

 of A.D to the square of the diameter of the bore of 

 CB. 



Another contrivance for enlarging the scale of the 

 barometer, is represented by Fig. 10. ABD is a 

 bent tube of the common diameter, terminated at the 

 upper extremity by the bulb A, in order that the as- 

 cents and descents of the mercury may be chiefly hi 

 the leg BD. EG is an index moveable about F as a' 

 centre ; DC is a float of ivory or glass, attached by a 

 slender wire to the extremity E of EG, and which, by 

 its ascent and descent on the surface of the mercury, 

 communicates motion to the index. By this means 

 the scale is enlarged in the ratio of FG to EF ; but 

 the friction is considerable, so that little reliance can 

 be placed on the degrees pointed out by the index. 

 It may be useful, however, merely to point out 

 changes in the atmospherical pressure, where the real 

 amount of these is of little importance. 



We shall conclude the account of the methods that 

 have been proposed for enlarging the extent of the 

 barometrical scale, by a description of a barometer in- 

 vented for that purpose by Mr Rowning. 



In Fig. 11. ABCD is a cylindrical vessel filled 

 with a fluid to the height W, in which is immersed 

 the barometer SP, consisting of the following parts : 

 The principal one is the glass tube TP, (represented 

 separately by tp,) whose upper end T is hermetically 

 sealed ; this end does not appear to the eye, being re- 

 ceived into the lower end of a tin pipe GH, which 

 in its other end G receives a cylindric rod or tube 

 ST, and thus fixes it to the tube TP. This rod ST 

 may be taken off, in order to substitute for it a longer 

 or a lesser, as occasion may require. S is a star at 

 the top of the rod ST, and serves as an index by 

 pointing to the graduated scale LA, which is fixed 

 to the cover of the vessel ABCD. MN is a large 

 cylindrical tube made of tin, (represented separately 

 by m n,) which receives into its cavity the smaller 

 part of the tube TP, and is well cemented to it at 

 both ends, that none of the fluid may get in. The 

 tube TP, with this apparatus being filled with mercu- 

 ry, and plunged into the bason MP, which hangs by 

 two or more wires upon the lower end of the tube 

 MN, must be so poised as to float in the liquor con- 

 tained in the vessel ABCD ; and then the whole ma- 

 chine rises when the atmosphere becomes lhrhter, and 

 vice versa. Let' it now be supposed that the fluid 

 made use of is water ; that the given variation in the 

 weight of the atmosphere is such, that, by pressing on 

 the surface of the water at W, the surface of the mer- 

 cury at X may be raised an inch higher (reckoning 

 from its surface at P) than before; and that the 

 breadth of the cavity of the tube at X and of the ba- 

 son at P, are such, that, by this ascent of the mercury, 

 there may be a cubic inch of it in the cavity X more 

 than before, and consequently in the bason a cubic 

 inch less. Now, upon this supposition, there will be 

 a cubic inch of water in the bason more than there 



Barometer. 



Barometer 

 with a 

 lever. 

 Plate 

 L1II. 

 Fig. 10. 



Rowning's 

 barometer; 

 Plate 

 LIII.. 

 Fig. 11. 



