152 



METEOROLOGY. 



Of the ane 



momctcr. 



Leslie's 

 anemome- 

 ter. 



Of the rain' 

 gge. 



PLATE 

 CCCLXXIV. 

 Fig. 7. 



Imperfec- 

 tion of the 

 instru- 

 ment. 



discovery of such an instrument on simple and accurate 

 principles, could hardly fail greatly to extend the boun- 

 daries of meteorological science. That science indeed 

 must always be in an imperfect state, so long as we 

 have not the means of measuring the effects of the most 

 active of all agents in the production of atmospherical 

 phenomena. 



The only other instruments connected with meteoro- 

 logy which we shall at present notice, are the Anemo- 

 meter, Rain-gage, and Atmometer. The anemometer, or 

 instrument for measuring the velocity of the wind, has 

 already been treated of under that article, where its 

 various forms have been fully described. Of these, 

 Lind's is perhaps the most convenient, though Leslie's 

 is unquestionably the most philosophical. The former 

 measures the force of the wind by the height to which 

 it raises a column of water in a bent tube, as indicated 

 by an attached scale. A table shewing the velocity of 

 the wind, corresponding to various heights in the co- 

 lumn, is given at the end of the article ANEMOMETER, 

 from which it appears, that when the velocity does not 

 exceed six miles an hour, the elevation of the water is 

 so small as to render it impossible to ascertain the pre- 

 cise height of the column without the help of a vernier, 

 a contrivance of which the nature of the instrument 

 hardly admits. We have found, indeed, from experi- 

 ence, that it is quite impracticable with this instrument 

 to determine the velocity of the wind with any thing 

 like accuracy, unless it amounts to about 1 1 miles an 

 hour, or what Lind calls a fresh breeze. 



Leslie's anemometer is founded on the principle, that 

 the cooling power of a stream of air is proportional to 

 its velocity. It consists simply of a common spirit of 

 wine thermometer having a pretty large bulb ; and the 

 force of the wind is deduced by an easy calculation, (see 

 ANEMOMETER,) from the time which it takes to cool 

 through half the number of degrees which it had been 

 previously raised by the heat of the hand. The result 

 thus obtained, we have found in a great many instances 

 to coincide almost exactly with that given by Lind, ac- 

 cording to Button's calculations ; the coincidence being 

 always closest when the wind was so high as to admit 

 of Lind's anemometer being accurately observed. It is 

 to be regretted that the instrument has hitherto been 

 so seldom used. 



The Pluvimiier, or rain-gage, for ascertaining the 

 depth of rain that falls on any given spot, is variously 

 fitted up; but we shall confine ourselves to the de- 

 scription of one which we have used for many years, 

 and which appears to us as simple, and at the same 

 time as accurate, as any that we have yet seen. It con- 

 sists of a copper funnel AB, (Fig.Y,) five inches in di- 

 ameter at top, and inserted into a tube CD of the same 

 metal thirty inches in length, and one and a half in dia- 

 meter, furnished with a stop cock E at the lower end. 

 It is examined every morning at ten o'clock, and if any 

 rain has fallen during the preceding 24 hours, it is mea- 

 sured by letting it off through the stop-cock into FG, 

 a glass tube of .5 of an inch in diameter, with an at- 

 tached scale of inches and tenths. By this means the 

 rain that falls on a circular area of 5 inches in diameter 

 is collected on an area of .5 of an inch, so that inches 

 and tenths of water in the tube, correspond to hun- 

 dredths and thousandths of an inch of rain on the sur- 

 face of the ground. 



To prevent waste by evaporation, the communica- 

 tion between the funnel and the copper tube is made 

 very narrow. The upper edge, or rim of the funnel, is 

 also turned upwards from the inclined direction of the 



under part, so as to stand perpendicular, the better to Meteoroio. 

 prevent the rain that falls within the gage from being Sf- 

 thrown over after striking against the interior. In V """V"" 

 some rain-gages, the rim is inclined a little from the 

 perpendicular inwards. This we conceive to be wrong, 

 as part of the rain that falls on the outside of the in- 

 clined rim may thus be thrown into the gage, which 

 will consequently indicate a greater quantity than it 

 ought to do. After all, however, the rain-gage is but 

 an imperfect instrument. It gives only an approxima- 

 tion to the depth of rain in any case, and of small quan- 

 tities it gives no indication at all, owing to the loss by 

 evaporation before passing from the funnel into the 

 tube. It is hardly necessary to remark, that the upper 

 edge of the funnel ought to be parallel to the horizon, 

 and that the instrument should be placed at a distance 

 from any object that might screen it from the rain. 



The Atmometer, or as it is sometimes called the evapo- Of the at- 

 romeler, for measuring the quantity of water evaporated mometer. 

 in any given time, is an instrument which has not till 

 lately been brought to any great degree of perfection. A 

 circular basin of uniform width from top to bottom, fill- 

 ed with water, is the simplest form of the instrument ; 

 and the quantity evaporated may be very accurately as- 

 certained by means of a graduated glass tube, similar 

 to that described above, for measuring the depth of 

 rain. There are several difficulties, however, attending 

 the use of the instrument, which render it a very un- 

 certain indication of the absolute quantity evaporated. 

 If it be exposed freely to the sun and wind, the heat 

 acquired by the vessel itself will rapidly promote eva- 

 poration ; and if the basin be kept nearly full, as it 

 ought to be, the water will be agitated, and thrown 

 over by the wind. Nor do we conceive, even though 

 this last accident were guarded against, that the result 

 would be either satisfactory or important. If the ob- 

 ject of the atmometer be to ascertain the dissolving 

 power of the air, it may be accomplished by placing 

 the basin in a sheltered situation as well as if it were 

 freely exposed, and if the quantity evaporated from the 

 ground be required, the simplest and the most accurate 

 method is that adopted by Dalton, as explained under 

 the article EVAPORATION. 



An atmometer, upon a very simple principle, was Leslie'* at- 

 some years ago constructed by Mr. Leslie. It consists of momeler. 

 a ball of porous earthenware two or three inches in di- 

 ameter, into which is inserted a glass tube, so graduat- 

 ed that the quantity of water contained between two 

 divisions of the 'tube, would cover the outer surface of 

 the ball to the depth of one thousandth part of an inch. 

 The ball and tube being filled with water, the top of 

 the latter is covered with a brass cap, which by means 

 of a screw and collar of leather is made quite air-tight, 

 and the instrument is suspended out of doors, freely 

 exposed to the wind. In this state the humidity exudes 

 through the surface of the ball just as fast as it eva- 

 porates, and the descent of the column in the tube in- 

 dicates the quantity evaporated. As the pressure of 

 the atmosphere is, in a great measure, removed by the 

 tightness of the collar, the water is prevented from pass- 

 ing through the ball so quickly as to drop, while the 

 space which it leaves empty at the top is occupied by 

 the very minute stream of air which is imbibed by the 

 moisture on the outside, and may be seen rising through 

 the water in the tube. 



The instrument is very simple, but we have found, Inappliea- 

 from numerous trials, that it is totally useless for a con- ble to mc- 

 tinued series of observations. The obvious impossibi- 'e r logical 

 lity of using it in frosty weather is itself an insuperable P ur P s 



