291 



BAROMETE R. 



Barometer, was before, because the water will occupy the space 

 v ^ _ v which the mercury has left. The whole machine will 



therefore be rendered heavier by the weight of a cubic 

 inch of water; and, by the laws of hydrostatics, will 

 sink till a cubic inch of that part of the rod WS, 

 which was above the surface of the water at W, 

 comes under it. Here if we suppose this rod so Ml ,11 

 that a cubic inch of it shall be 1 [ inches in length, 

 the whole machine will sink 14 inches lower in the 

 fluid than betore ; and consequently the surface of 

 the mercury in the bason will lie pressed, more than 

 it was before, by a column of water 14 inches high. 

 But the pressure of 11 inches of water is equal to 

 one of mercury ; and this additional pressure will 

 make the mercury ascend at X, as much as the sup- 

 posed variation in the weight of the air did at first. 

 This ascent will make room for a second cubic inch 

 of water to enter the bason ; the machine will there- 

 fore be again rendered so much heavier, and will sub- 

 side 14 inches farther, and so on in infinitum. If less 

 than 14 inches of the rod be sufficient to make a cu- 

 bic inch, the scale of variation will be finite, and may 

 be made in any proportion to the common one. 



Mr Rowning never actually constructed a barome- 

 ter according to the above principles, nor, so far as 

 we know, has it been executed by any other person. 

 It might, no doubt, point out very minute changes 

 on the weight of the atmosphere ; but the difficulty 

 of adapting a scale to it, would render it of little prac- 

 tical utility. 



It is extremely desirable, for meteorological pur- 

 poses, to have a regular and successive series of the 

 changes which take place in the pressure of the at- 

 mosphere during any given period ; but as this would 

 require constant attendance on the part of the obser- 

 ver, mechanical contrivances have been adopted for 

 registering the indications of the barometer, and re- 

 taining them in a connected form : When the instru- 

 ment is fitted up in this manner, it is called a self-re- 

 gistering barometer. 



The most simple kinds of self-registering barome- 

 ters are such as indicate the greatest rjseand fall of the 

 mercury, or it extreme range, during any stated pe- 

 riod ; and when this only is required, the object is easily 

 accomplished. Of this description is the self-regis- 

 tering barometer, invented by Alexander Keith, Esq. 

 F. R. S. Edinburgh. It consists of a bent tube, such 

 Plate as ABD, Fig. 12, hermetically sealed at A. The 

 1.1II. mercury in the shorter leg supports a float, to which 



fig. 12. j s affixed a slender wire terminating in abend or knee. 

 This knee embraces a very small wire stretched along 

 the scale, and pushes upwards or downwards two bits 

 of glazed silk which slide along the wire very easily, 

 yet so as to retain the position to which they are 

 moved by the ascent and descent of the mercury. 

 The instrument is prepared for experiment by bring- 

 ing the two bits of silk in contact with the bent knee 

 of the float -wire ; the points to which they may af- 

 terwards be removed, indicate the extreme range of 

 the mercury during the interval of any two observa- 

 tions. 



When not only the greatest and least altitude of 

 the mercury is sought for any given time, but also its 

 precise height at every intermediate moment, more 

 omplicated contrivances must be employed : the in- 



Self-regis- 

 tcring ba- 

 rometers. 



stnur.ent must then consist of a barometer connected 

 with a time-piece, and a crayon or pencil affix 

 float obeying the mbtiont of the mercury. The 

 t number of self-registering barometers of tins 

 nature are so constructed, thai the crayon is made to 

 describe a continuous line on a vertical cylinder, turn- 

 ing on its axis by means of clock-work, and making a 

 certain number of revolutions in some stated tune. 

 The Cylinders are divided longitudinally by parallel 

 into equal spaces, corresponding to some parti- 

 cular portion of time ; and thus the line described by 

 the crayon in that time, indicates the successive 

 heights of the mercury during its continuance. 



M. d'Ons-en-Bray was the first who applied the 

 pendulum to meteorological instruments ; but, in 

 every contrivance which has been adopted, the great 

 friction arising from the traces of the crayon pre- 

 vents, in a considerable degree, the free motion of 

 the mercury, so that the indications of the register 

 are little to be relied on. The description of ano- 

 ther instrument of a more improved construction, in- 

 vented by M. Changeux, will be seen in the NowoeUei 

 tie la Republique des Letlres et des Arts, par M. 

 Blancherie 1779, p. 134, 167, 170, 187; and 1781, 

 p. 30. 



The principle proposed by Dr Brewster, in the 

 article Atmospherical Clock, for measuring the 

 mean temperature of the atmosphere, during any gi- 

 ven interval, may also be employed in the construc- 

 tion of a barometrical clock ; by which the average 

 height of the barometer, during any given time, will 

 be indicated on the dial plate. The same construc- 

 tion is applicable to the hygrometer. 



We shall conclude this article with a descrip- 

 tion of several instruments which have been sug- 

 gested for measuring the pressure of the atmosphere, 

 which though founded on just principles, are rather 

 curious than useful. 



The conical or pendant barometer, invented by M. 

 Amontons in 169.5, consists of a truncated conical 

 tube, hermetically sealed at one end, and suspended 

 in a vertical position. It has no cistern ; the conical 

 figure of the tube, and the smallness of the bore, ren- 

 dering that unnecessary. The length may be varied 

 at pleasure, and will depend on the conical form of 

 the bore ; so that the slower the degree of contrac- 

 tion, the more extended will be the scale. Thus 

 suppose 28 inches of mercury in the lower part of 

 the tube occupied 31 inches, 80 inches higher the 

 range would be 80 28, or 52 inches. 



This instrument may be employed as a marine ba- 

 rometer ; but the friction of the mercury is so great, 

 owing to the bore of the tube being necessarily very 

 small, that it is seldom used. 



The statical barometer of Otto de Guericke, Boyle, 

 and others, consisted of a large hollow sphere fixed 

 at one end of the arm of a delicate balance, counter- 

 poised by a weight of brass at the opposite. These 

 two bodies being of the same weight, but of different 

 volumes, if the fluid or medium in which they are 

 suspended becomes more or less dense, an apparent 

 change of weight will take place, and the equili- 

 brium will be subverted. If the air becomes heavier, 

 the hollow ball will appear to become lighter, as it 

 will lose more of its weight than the counterpoise. 



Barometer; 



Barometri- 

 cal clock. 



Conical 

 barometer. 



Statical 

 barometer 

 of Otto de 

 Guericke, 

 Boyle, &c. 



