290 



BAROMETER. 



Plate 

 LIV. 

 Fig. 4. 



Kg. 5. 



Barometer, round with a smooth and steady motion, for the pur- 

 ' ' /"" pose of placing the instrument in the best light for 

 reading off, &c. The actually divided scale com- 

 mences at about 15 inches above the zero, and is con- 

 tinued as high as 33 inches, and, by the usual help of 

 a vernier, is subdivided down to .001 of an inch. A 

 longitudinal slit, from end to end of the divided part, 

 exposes to view the glass tube and mercury within it. 

 The whole of this part consists of two tubes of brass. 

 In the inside of the interior one, slides a cylindrical 

 piece, on which is divided the vernier scale, the index 

 to which is the lower end of the piece. In taking 

 the height of the mercury, this piece is brought down 

 so as just to exclude the light from passing between 

 itsejf and the spherical surface of the mercury. The 

 screw at top, although but a short one, performs 

 this office in whatever part of the scale the vernier 

 piece may be ; for it acts upon the interior long tube, 

 in the inside of which, the piece is sustained by 

 friction ; and in which, it is, on every occasion, 

 to be set by hand nearly. The tripod is altogether 

 similar to what Mr Ramsden used for the same 

 purpose, as far back, perhaps, as the year 1775. 

 It affords, when closed, ( Fig 4.) a safe and convenient 

 packing-case for the instrument. The structure of 

 the staff head is curious. The principal part is a 

 circle (Fig. 5.) about .75 of an inch broad, jointed 

 in three pieces : these, although they seem in prin- 

 ciple to be incapable of motion, yet, in practice, 

 produce what is fully adequate to the purpose. 

 The three joint-pins extend inwards, so as to pass 

 through a circular rim, which they hold fast : with- 

 in this rim is hung a similar one by two pivots; and, 

 inside the latter, at right angles to the pivots, are 

 fastened two Y's or angles, in which the barometer 

 hangs by its gudgeons. Thus are brought about, in 

 a small compass, the means of extending the legs, of 

 turning the instrument about respecting the tripod, 

 and an universal joint, whereon it readily places it- 

 self perpendicular to the horizon. 



We shall conclude the account of the portable baro- 

 meter for the measurement of heights, with the descrip- 

 tion of one which has lately been constructed by Miller 

 and Adie, uniting the advantages of Troughton's 

 with several important improvements. The general 

 appearance of this instrument, when on its tripod, 

 is nearly the same as that represented in Fig. 3. The 

 scale and vernier are the same as those already de- 

 scribed ; the top of the tube, which forms the frame, 

 is cut open on the opposite side, so as to allow the 

 finger and thumb to lay hold of the head of the ad- 

 justing screw, and is continued beyond it about half 

 an inch ; this space is occupied by a circular level, 

 by which the instrument is very conveniently placed 

 in a vertical position. The ball of the attached tker- 

 mometer is made of a piece of the barometer tube 

 about 1.5 inches long, and bent back so as to lie pa- 

 rallel to the tube of the barometer, with which it is 

 nearly in contact. The mercury in botli being thus 

 similarly situated, is equally and simultaneously af- 

 fected by a change of temperature. The cistern is 

 made of two circular pieces of wood, connected to- 

 gether with leather. Two concentric screws work 

 in the bottom of the external brass cover : the outer 

 screw is intended to raise the whole bottom of the 

 1 



Miller and 

 Adie's ba- 

 rometer. 



Plate 

 LIV. 

 Fig. 6. 



cistern, in order to preS9 the mercury quickly to the Barometer, 

 top of the tube, either for carriage, or to make room v 

 for a considerable descent of the mercurial column. 

 The use of the small central screw is to adjust the 

 surface of the mercury to zero, by pressing 3 small 

 leather b.ig in the bottom of the cistern, by which 

 the adjustment is performed more steadily than if the 

 whole area were acted upon. In the top of the cistern 

 is inserted a bit of the barometer tube, about half an 

 inch long, surrounded by a hollow cylinder of ivory, 

 terminating in a female screw, which is stopped by a 

 finger screw of ivory, to prevent the escape of the 

 mercury when the instrument is packed up for car- 

 riage. The ivory cylinder has an external covering 

 of brass, whose width is equal to the diameter of the 

 tube that forms the frame of the barometer ; both 

 the brass and ivory coverings are cut open on the 

 opposite sides, that the surface of the mercury may 

 be seen through the glass tube, and that the line oi 

 light betwixt it and the upper edges of the slits in 

 the brass cover, which is the beginning of the scale, 

 may be distinctly cut off. By this method of ad- 

 justment, the upper and under surfaces are observed 

 as nearly as possible under similar circumstances, as 

 to inflection of light, attraction, Sec. In this con- 

 struction, the tube being readily accessible, can easily 

 be cleaned when it is soiled by the mercury, which 

 always happens when the surface of the metal is ex- 

 posed to the action of the air. 



On 'the top of the tripod is a hollow ball and 

 socket ; in the centre of the ball is an universal joint, 

 in which the barometer is hung in Y's ; it is then set 

 perpendicular by the level on the top. Four finger 

 screws hold the frame of the barometer steady so as 

 not to be shaken by the wind, or hand, in turning the 

 adjusting screw at the top. When the instrument is 

 placed on uneven ground, so as to require an adjust- 

 ment beyond the range of the screws, it is brought 

 nearly perpendicular, and to the most advantageous 

 position for observing, by unturning a little the mil- 

 ling forming the upper part of the socket, which 

 must be again pinched. The tube is never removed 

 from the legs of the tripod ; to prepare it for carriage, 

 the pivots on which it hangs are lifted out of the Y's, 

 and turned a little to one side; and it is then let 

 down until the pivots, which project from the frame 

 about an inch and a half below the top, rest on the 

 ring at the upper opening of the ball. The legs are 

 then closed on it, and held together by brass rings. 

 A brass cap is then screwed on above the joints of 

 the legs, which protects all the upper parts. Two 

 microscopes, for observing the surfaces of the mer- 

 cury, pack in the lower part oi the legs. Fig. 6. 

 represents the cistern, and the top of the tube. 



When the barometer is to be used at sea, some 

 contrivance is necessary to prevent the oscillations of 

 the mercury. Two methods are employed for this 

 purpose : cither to prevent, by mechanical means, the 

 vibrations of the instrument itself, or to check the 

 motions of the mercury in the tube by some pecu- 

 liarity of' form. Both methods may also be con- 

 joined. 



One of the earliest marine barometers with which 

 we are acquainted, was suggested by Dr Hooke. 

 It consited of two thermometers, or rather of a ma- 





Plate 

 LIV. 

 Fig. 6. 



Marine ba- 

 rometers. 



Hooke's 



marine 



barometer. 



