i 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 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 ther 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 both 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 nether 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 cistern, in order to press the mercury quickly to the top of the tube, either for carriage, or to make room 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 a small leather hag 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 of 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, &c. 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.
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 fur 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 arc 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 of the legs. Fig. 6. represents the cistern, and the top of the tube.
When the barometer is to he used at sea, some ; contrivance is necessary to prevent the oscillations of the mercury. Two methods are employed for this purpose : either 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 arc acquainted, was suggested by Dr Hooke. It consited of two thermometers, or rather of a ma nometer, (See MAsoacTEn), and a thermometer placed together in the same frame. The thermome ter was affected, onlv by the warmth of the air ; the manometer, acting by the expansion and contraction of an included bubble of air, was affected not only by the warmth, but also by the weight of the air. If the two tubes, therefore, are so graduated as to agree with each other when the air is included, it is evident, that when they afterwards agree, the pres sure of the atmosphere must be the same as when the air was included. And in general, if the thermome ter be taken as a standard, the difference of ascent or descent in the other will point out the increase or de crease of the weight of the air. At the same time, it ought to be mentioned, that the condensation and rarefaction of the air, on which this instrument is al together founded, do not depend solely on the weight of the atmosphere, but are greatly influenced by tem perature. Hence this instrument cannot, strictly speaking, be called a barometer, but rather a con trivance for pointing out alterations in the state of the air ; and as such,-according to the observations of Dr Halley, it may be of considerable_ utility. During his voyage to the South Sea, he had one of these barometers, and 66 it never failed," says he, to prognosticate, and give early notice of all the bad weather we had, so that I depended thereon, and made provision accordingly; and from my own experience, I conclude, that a more useful contrivance bath not for this long time been offered for the be nefit of navigation." Phil. Trans. 1700-1, No 269, p. 791.
M. Passement obviated the effects of the motions of a ship at sea on the barometer, by twisting the middle of the tube into a spiral consisting of two con volutions ; by this contrivance, the impulses which the mercury receives, mutually destroy each other, by acting in opposite directions. The effects of the external and momentary impulses may also be dirni.: nished by widening the upper part of the tube where the scale is applied. The oscillations, which would be very perceptible in the tube, become scarcely sen sible when they are thus diffused over a larger ex tent of surface.