150 



METEOROLOGY. 



Mcteorolo- the water will follow it, and continue to do so till it 

 ey- rises to the height of about 33 feet. The same thing 

 1 "~Y""* w jH take place if the tube be immersed in mercury, 

 with this difference, that the mercury will ascend only 

 to about the height of 30 inches. The effect in both 

 cases is produced by the weight of the atmosphere. 

 l?y drawing up the piston, the pressure of that weight 

 is removed from the portion of the fluid immediately 

 under it, while it continues to be exerted on the sur- 

 face of all the rest. The liquid is therefore forced up 

 into the tube, till the column be of such a height as to 

 balance the weight of the atmosphere. If a glass tube 

 of convenient length, and open at one end, be filled 

 with mercury, and then inverted perpendicularly into 

 a basin of mercury, so that the open end may be 

 under the surface, the mercury will sink down from 

 the upper extremity, and stand at the point to which 

 it would have risen, had a piston been employed as 

 stated above. In this form it constitutes a barometer, 

 and the changes of weight which the atmosphere un- 

 dergoes, are indicated by the rising and falling of the 

 mercury in the tubs. To measure these variations, a 

 scale may be placed parallel with the tube, and divid- 

 ed into inches and decimals, beginning at the surface 

 Inaccuracy of the mercury in the basin. It is obvious, however, 

 in the com- that if the scale be fixed, when the mercury sinks in 

 tne tube, the surface of that in the basin must rise in 

 proportion to the relative width of the basin and the 

 tube, and consequently stand higher than the com- 

 mencement of the scale; and in like manner, when the 

 mercury rises in the tube, the surface of that in the 

 basin must sink, and stand lower than the beginning 

 of the scale. In the one case therefore the mercury in 

 the tube will stand at a higher, and in the other at a 

 lower point than it ought to do. This inaccuracy is 

 in part removed, by making the basin very wide re- 

 latively to the tube, but -still more effectually by con- 

 structing it in such a manner that the surface of the 

 mercury in the basin may be raised or depressed to the 

 commencement of the scale. In general a portion on- 

 ly of the scale is divided, viz. from the twenty-seventh 

 to the thirty-first inch, reckoning from the surface of 

 the mercury in the basin, because these are found to be 

 the ordinary limits of the barometrical range. See 

 BAROMETER. 



Situation of It is hardly necessary to observe, that the barometer, 



the baro- j n order to indicate correctly the weight of the atmo- 



3 be sphere, should be placed in a perpendicular position. 



' '. n | It should also have a thermometer attached to it, to be 

 account in ... u? 



recording observed at the same time with the barometer itself, as 

 its indica- the length of the column of mercury is affected not 

 tions. only by the pressure, but also, though in a much 

 smaller degree, by the temperature of the atmosphere. 

 Thus, if the barometer at the temperature of 32 stands 

 at 30 inches, it would, under the same pressure at the 

 temperature of 62, stand at one-tenth above 30 inches. 

 The elevation of the place where the barometer is si- 

 tuated, is also necessary to be taken into the account. 

 When carried upwards, the mercury is found to sink 

 in the tube at the rate of about one-tenth of an inch 

 for every 90 feet of ascent, (see PNEUMATICS) and con- 

 sequently no comparison can be instituted between ob- 

 servations of the barometer at different places, unless 

 the heights of these places be accurately known. 

 Self-regis- Several plans have been proposed, and attempts 

 tering baro- made, to construct a self- registering barometer, for re. 

 meter. cording the greatest elevations and depressions of the 

 mercury, in the absence of the observer. The con- 

 trivances are abundantly simple, but they are all liable 



mon baro 

 meter. 



Methods 

 employed 

 for correct- 

 ing it. 



CC( f L3 f xlr 



to the same objection, that by increasing the friction, Meteorolo- 

 they diminish the delicacy of the instrument, thereby ^ gy- 

 rendering it unfit for indicating minute changes in the 1 *""Y"""' r 

 pressure of the atmosphere. The simplest, and per- 

 haps the best contrivance of this sort hitherto proposed, 

 is described in our article BAROMETER. For represen- 

 tations of different forms of the barometer, see Plates 

 LIII. and LIV. 



An improved portable barometer, or rather a substi- Adie's 



tute for that instrument, has lately been invented by sympie- 



Mr. Adie of this place, which we shall shortly describe someter. 



in the words of the inventor himself, as stated in the 



patent. The principle of the instrument, which Mr. 



Adie has denominated a Sympiesometer, consists in 



measuring the weight of the atmosphere by the com- 



pression of a gaseous column. It consists of a tube of 



glass A BCD, (Fig. 4.) of about 18 inches long, and 0.7 Fig. 4. 



of an inch diameter inside, terminated above by a bulb 



D, and having the lower extremity bent upward, and 



expanding into an oval cistern A, open at top. The 



bulb D at the upper extremity being filled with hydro- 



gen gas, and a part of the cistern A and tube BC with 



almond oil, coloured with anchusa root, the enclosed 



gas, by changing its bulk according to the pressure of 



the atmosphere on the oil in the cistern, produces a 



corresponding elevation or depression of the oil in the 



tube, thereby indicating the variations in the weight 



of the atmosphere. The scale for measuring these 



changes is determined by placing the instrument 



along with an accurate barometer and thermometer, 



in an apparatus where the air may be condensed or ra- 



rified, so as to make the barometer stand at 27, 28, 29, 



30, or any other given number of inches. The differ- 



ent heights of the oil in the tube of the sympiesometer 



corresponding to these points being marked on its scale 



EF, and the spaces between being divided into an 



hundred parts, these divisions correspond with hun- 



dredths of an inch, on the scale of the mercurial baro- 



meter. To correct the error that would arise from the 



change produced in the gas by a change of tempera- 



ture, the principal scale EF is made to slide upon 



another GH, so graduated, as to represent that change, 



and corresponding to the degrees of a common ther- 



mometer IK attached to the instrument. In this state, 



the rule for using the instrument is simply to observe 



the temperature by the thermometer, and to set the 



index a which is upon the sliding scale, opposite to the 



degree of temperature upon the fixed scale, then the 



height of the oil, as indicated on the sliding scale, will 



be the pressure of the air required. The sliding scale 



EF is moved by means of the knob L. We shall have 



occasion in another part of this article to notice the in. 



dications of the sympiesometer, as compared with 



those of the common barometer. See the Edinburgh 



Philosophical Journal, vol. i. p. 54. 



The Hygrometer is an instrument of great import- Of the hy 

 ance in meteorology, as indicating changes in the hu- grometer. 

 midity of the atmosphere. The idea of this instru- 

 ment seems to have been first suggested by the expan- 

 sions and contractions which animal and vegetable 

 substances are observed to undergo, when exposed to 

 different degrees of moisture. We find accordingly 

 that a great proportion of the hygrometers hitherto em. 

 ployed, have consisted of one or other of these sub- 

 stances, and though some of them are susceptible of 

 great delicacy, they are all radically defective, inas- 

 much as the frequent changes to which they are ex- 

 posed must in time produce a partial derangement in 

 the texture of the substances, and consequently their 



