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 t pi) .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. raduated scale LA, which is fixed to the cover of the vessel ABCD. MN is a large cylindrical tube made of tin, (represented'separately by ni 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 basal) 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 a and then the whole ma chine rises when the atmosphere becomes lighter, and vice versa. Let it now he 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 ha. son at P, are such,that, by this ascent of the mercury,' there may be a cubic inch of it in the cavity X than before, and consequently in the bason a cubicf inch less. Now, upon this supposition, there will be. a cubic inch of water in the baton more than there was before, because the water will occupy the space 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 \VS, which was above the surface of the water at AV, comes under it. Here if we suppo'se this rod so small that a cubic inch of it shall be 14 inches in length, the whole machine will sink 14 inches lower in the fluid than before ; and consequently the surface of the mercury in the bason will be pressed, more than it was before, by a column of water 14 inches high. But the pressure of 14 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 idinitunt. 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 sey:rc oistering barometer.
The most simple kinds of self-registering barome ters are such as indicate the greatest rip and fall of the mercury, or its-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 as ABD, Fig. 12, hermetically sealed at A. The mercury in the shorter leg supports a float, to which is affixed a slender wire terminating in a bend 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 r-omplicated contrivances must be employed : the in ailment must then consist of a barometer connected with a time-piece, and a crayon or pencil aifixed to a float obeying the motioas of the mercury. The greater number of self-registering barometers of this nature are so constructed, that 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 lines 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 he seen in the Nouvelles, de la Republique des Lettres 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 ATMOSPIIERICAL 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 dock ; by which the average height of the barometer, duffing 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 1695, 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 high,n- the range would be 80-28, or 52 inches.