Besides these lineal divisions, the scale is divided into other points, having a reference to the instrument in its capacity of indicating the probable state of the weather. At 31 inches, the highest point of the scale of variation, set fair is marked on the one side, and set frost on the other. At 30 inches, fair is written, in like manner, on the one side, and frost on the other ; and at half an inch below is written the word change able, which answers both for summer and winter.
The common barometer answers sufficiently well for most purposes, when the observations are made at the same place ; but as many of these necessarily imply a change of situation, it soon became an object of importance to construct barometers in such a manner that they might be conveyed from one place to another without much inconvenience or risk. Barometers of this kind, which are called portable barometers, are chiefly employed for measuring heights. They have assumed, under the hands of dif ferent artists, a variety of forms.
Derham mentions a portable barometer ; but as many circumstances connected with the accuracy of the instrument, are entirely overlooked in its con struction, it does not merit a particular description. (Phil. Trans. vol. xx. N° 236.) The portable ba rometer consists, in general, of a tube of the usual length, passing through the upper part of a wooden cistern, to which it is glued, and the bottom of which is made of leather. The tube being filled with mer cury, which has been previously well purged of air, and placed in a proper position, the superfluous men: cury descends into the cistern, and assumes a level in the tube, corresponding to the weight of the ex ternal air. The surface of the mercury in the cistern is adjusted to the same level by a screw, which presses more or less against the flexible leather at the bot tom, and raises or depresses it at pleasure. From the line of this level, which is called zero, the scale commences, and is reckoned upwards to the height of about 32 inches : the actual divisions of the scale begin at ab mt 15 inches. Various methods have been em ployed for constructing the portable barometer in 4 manner best suited to carriage, to placing the tube in a vertical position at the time of observation, to ascertaining the surface of the mercury, and to ma king allowances f'or the expansion and contraction of the mercury by changes of temperature: but it will be sufficient to-describe the instrument in its pre sent most improved form.
Figures 1, 2, and 3, represent the portable barome ter as constructed by Mr Troughton, and first made t by him in 1785. The greatest peculiarity in this t instrument, according to the opinion of this ingenious and philosophical artist, consists in the excellent 1 manner in which the mercury in the cistern is set to the zero of the scale of inches. For this purpose, a glass cylinder of about 2.5 inches diameter, and as much in length, contains the mercury. An external covering of hollow brass, terminating in a female screw a little above and below the glass, admits male screw pieces, whose ends, well leathered, being pres sed hard against the ends of the glass, prevent the escape of the fluid. Near the upper end of the brass
cover are two slits made horizontally, one before,• and the other behind, exactly similar and opposite to each other. At bottom is a screw, seen better in the section Fig. 2. which, acting upon the usual leather F bag, forces the quicksilver upwards at pleasure, and, by filling every part, renders the instrument portable. But the primary design of this screw is, to furnish the means of adjusting the surface of the mercury in the glass cistern, so as just to shut out the light from passing between it and the upper edges of the slits in the brass cover. This is the mode of adjusting to zero ; and it follows, that the upper edges of the slits must represent the beginning of the scale of inches. The frame is entirely made of a brass tube, and above the cistern is of about 1.1 inch diameter. The first ten inches of the lower end is occupied by•a thermo meter, whose bulb, bent inwards, is concealed within the frame. At about three inches higher, it attaches to the stand by a ring, in which the frame turns 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 itself 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.) abbut .75 of an inch broad,. jointer; 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.