THERMOMETER AND PYROMETER, 



45 



valuable indicator of the state of the 

 air favourable to the deposition of that 

 interesting meteor. Such is its extreme 

 delicacy, that, w hen rising, its progress 

 is checked by the smallest cloud sailing 

 over it ; and it may be kept in a state of 

 oscillation, by being alternately stretched 

 beyond the edge of a parasol, and drawn 

 within its shade, when the sky is clear 

 and serene. 



Besides the forms given above, Leslie 

 describes two others, the Standard and 

 the Sectorial. The first has the stem 

 bent up as in the hygrometer, and one 

 of the balls covered with silver leaf or 

 gilt, near the side of the cup c, c, with 

 the naked ball in the centre, as the 

 balls a, b, in A. The second has the 

 cup, c, c, formed with a notch in its 

 bottom, for admitting a partial vertical 

 motion round the ball b. The motion 

 is given by means of a toothed sector 

 and pinion ; from which the name is 

 derived. This form is applicable to 

 ascertain the radiation from the earth, 

 when we ascend a mountain, or rise in 

 a balloon. 



7. The differential hygrometer and 

 photometer would have come into 

 more general use had they indicated the 

 maximum and minimum between any 

 two times of observation. In their pre- 

 sent construction they only show the 

 state of the atmosphere at the moment 

 of observation, and, therefore, require 

 an attention which few have leisure or 

 inclination to bestow on meteorolo- 

 gical observations. To render them 

 more extensively useful the following 

 alteration is suggested, by which they 

 are brought nearly to the thermoscope 

 of Rumford in form. The tubes con- 

 necting the balls have the upright part 

 of their stems shortened, and the hori- 

 zontal part extended : instead of a co- 

 loured fluid filling the stems, there is a 

 short column of mercury introduced 

 into the horizontal part of the stem ; 

 the motion of which, towards either ball, 

 carries before it a piece of steel w ire, 

 which constitutes the index of maximum 

 and of minimum change during the 

 absence of the observer. This con- 

 struction will be readily understood from 

 the figure 43, which represents the re- 

 Fig. 43. 



gister hygrometer. A double scale lies 

 along the horizontal part of the instru- 

 ment. When the indices are adjusted 

 for a fresh observation, they are brought 

 by a magnet to each extremity of the 

 little column of mercury.* 



CHAPTER V. 



Of some peculiar Applications of the 

 Thermometer. 



THERE are a few applications of the 

 thermometer to certain useful pur- 

 poses, which ought to find a place in 

 the history of the instrument ; I allude 

 particularly to the Statical Thermometer 

 of Dr. Gumming, the Balance Thermo- 

 meter of Mr. Kewley, the "Hygrometri- 

 cal Instrument of Mr. Daniell, and the 

 Barometrical Thermometer of Mr. 

 Wollaston. 



1. The Statical Thermometer of Dr. 

 Gumming, now of Chester, was con- 

 trived by that gentleman, in 1808, and 

 intended by him as a mode of opening 

 and closing windows and ventilators in 

 apartments, by the variations in tempe- 

 rature of the included air. This inge- 

 nious application of statical principles 

 was shown to numerous friends at dif- 

 ferent times, in his residence at Den- 

 bigh, and was afterwards, for a consi- 

 derable time, exhibited in the Denbigh 

 Dispensary. The general form of the 

 instrument is represented in Jig. 44, 

 where a b is a glass matrass, or a ball 

 and tube of iron ; the globular termi- 

 nation of which is capable of containing 

 four or five pints of air, and the tube 

 is about twenty-five inches in length, 

 and from one to two inches in diame- 

 ter. A portion of the tube is filled 

 with mercury ; and in this state it is 

 inverted, and its extremity plunged in 

 a cylindrical jar for containing the same 

 fluid. The ball is covered by a net of 

 strong cord, or of wire, which forms a 

 ring at the top, for the suspension of the 

 ball and tube. From this ring passes a 

 cord over the pulley, d; and it may 

 either pass upward under the pulley, e, 

 to be attached to the frame of a swing 

 window, as shown at g ; or downwards 

 over the pulley/, to be fixed to the ven- 

 tilator h. "When the heat of the apart- 

 ment expands the air in the ball, it de- 

 presses the mercurial column in the 

 tube, b ; by which the whole instrument 



* This instrument acts rather slowly, and the 

 motion of the indices is not quite smooth; but it ap- 

 pears capable of supplying a desideratum in me- 

 teorological observations a register hygrometer, 



