METEOROLOGY. 



151 



A elf-ro- 

 filtering 

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FlATT 

 CCCtMIT. 



F.J.A. 



Method of 



r * '- i t 

 UM hjirro- 

 mrtrical 



indication* become inaccurate. The only hygrometer 

 that we think deserving the attention of mcteorolr 

 is that delicate instrument the thermoscope of Count 

 HumtoTtl, or the differential thermometer of Mr. Leslie, 

 instrument has already been described, and its 

 superiority to every other pointed out, in the article 

 HYUROMBTRY, to which we refer*. We apprehend, 

 however, 'hat it is not yet so perfect as to be incapable 

 of farther improvement, and we have therefore to pro- 

 pace a modification of it, which we conceive is better 

 adapted to the purposes of meteorology. In the course 

 of our observations, we had long felt that a self-regis- 

 tering hygrometer was still a desideratum ; and though 

 it readily occurred to us that the differential thermome- 

 ter might easily be constructed, so as to mark the great- 

 est dryness in the absence of the observer, there ap- 

 peared to be some difficulty in making it to register the 

 greatest moisture. Both these objects are completely 

 accomplished by the following simple contrivance, 

 which, as far as we know, is the first attempt of the 

 kind that h-n hitherto been made. We have already 

 submitted it to the public through the medium of 

 Blackwood'f Magazine, (No. r-.v) though we do not 

 know that it has yet been adopted in any meteorologi- 

 cal observations. 



A tube, CD (Fig. 5.) such as in commonly used for 

 constructing a self-registering thermometer, is bent up- 

 wards at C and D, n i terminates in a bulb A. Into 

 this bulb is introduced a portion of sulphuric acid, suf- 

 oosit to fill the tube and a small part of the bulb ; and 

 along with the acid a small bit of glass a, of such a din- 

 neter at to move catily in the tube when the instrument 

 is inverted. To the extremity I) another bulb B is at- 

 tached ; and the air contained in both bulb* is so ad- 

 iwtod, that when they are at the same temperature the 

 liquid stands at a point near the extremity I), and which 

 is narked O on the attached scale FF. If the temper, 

 ature of the bulb B be now increased, or, which pro- 

 duces the ssune effect, if that of A be diminished, the 

 portion of air in the upper part of the bulb will con- 

 tract, while that contained in B will expand in the 

 SUM prsyortion, and the liquid will of course In- forced 

 from D towards C, the difference of temperature being 

 indicated upon UM scale in degrees, each of which, ac- 

 cording to the graduation adopted by Mr. Leslie, is the 

 thousandth part at UM difference between the tempera- 

 ture* of fret-zing and boiling water. The divisions of 

 which has been called the wiUetimal, may be 

 Let the bulb A be surrounded with 

 '. while the instrument is placed in an at- 

 of any higher temperature, say JO* of Fab- 

 id let the point be marked at which the li- 

 quid becomes stationary. The distance between aero 

 and this point, hi the case aupyeaad, will be 18 of 

 it, or 100* of the millesimal scale, and that 

 being divided into an hundred equal parts, 

 will give the graduation rMBsrad. The divisions may 

 be e*tadd beyond 100 if necessary, but in this clt- 

 mat* greater range will seldom be required. 



To prepare the instrument for observation, it only 

 remains to cover the bulb A with silk, and moisten it, 

 taking care that the two bulbs ho a* nearly as pomible 

 of the same colour. The index, or small bit of glais 

 a, it then to be brought to the surface of the liquid, by 



this scale, 



depressing the extremity D, and the instrument to be Meteorolo. 



exposed in a horizontal position. As the evaporation K. v - . 



from the surface of the bulb A goes on, the air within ' ""V 1 

 contracts, from the depression of temperature produced 

 by the evaporation ; and the liquid is forced from D to- 

 wards C by the elasticity of the air in B, carrying with 

 it the index a. When the evaporation has reached its 

 maximum, the liquid as well as the index becomes sta- 

 tionary ; but should the process of evaporation dimi- 

 nish, the liquid will a^ain move towards D, while the 

 index is left behind, thus marking (lie maximum ofdry- 

 neu in the absence of the obtervcr. 



To find the greatest degree of moisture, another in- PLAT* 

 strument is to be employed, which is> represented in ' 1 1 LXXl 

 Fig. fi. The only difference Ixlween this and the Fig. 6. 

 former is, that the air in the two bulbs is to be so ad- 

 justed, that when they are at the same temperature the 

 liquid may stand near the extremity C, the distance be- 

 tween C and. zero being a little more than the length 

 of the index a, and the bulb B is to be covered as A 

 was in the former. The scale is graduated as before. 



When the instrument is adjusted and exposed, eva- 

 poration goes on from the surface of B, and the liquid 

 of course moves towards D ; but when the evaporation 

 diminishes, the liquid is again forced back towards C 

 till it arrive* at the index a, and sheuld the evaporating 

 energy, or dryness of the air, still continue to dinuir-h, 

 the index itself is then carried towards zero till the eva- 

 poration be at its minimum. The liquid then becomes 

 stationary; and though it should afterwards mount 

 higher, in consequence of an increased evaporation, still 

 toe index remains at the lowest point to which the li- 

 quid had sunk, thus marking the minimum ofdn/nett in 

 the absence of the observer. In both cases, as in t he ori- 

 ginal farm of the hygrometer, the covered bulb is to be 

 kept continually moist with water, conveyed to it by 

 filafswnta of floea silk from an adjoining vessel. We 

 shall afterward* have occasion to notice the advantages 

 of the above form of the hygrometer. 



In oar meteorologies.! observations, we have been The indica- 

 accMtoined for several years to employ two delicate tion of the 

 thermometers instead of a hygrometer, and find that h >""" 

 they not only answer the same purpose, but have the jj*] ^ 

 additional advantage of showing the absolute as well as M ' ' \ Z 

 the relative temperatures of the two bulbs. The ther- two ther- 

 mometers are of such a range as to admit reading off' muineuri. 

 the tenth of a centigrade degree, which c: n -ponds 

 with a degree of Leslie's scale. The bulb of one of 

 them is covered with silk kept constantly moist, and 

 the difference of temperature lietween the two, express- 

 ed in tenths of a centigrade degree, gives the height of 

 the hygrometer by the millesimal scale. 



Of toe Electrometer as meteorological instrument, it Or the elec- 

 ts 1 oni 

 ft* 



to say much. It has never yet been re- trotncter. 



t for a limited period, nor is it 

 likely to be so soon. Our readers will find a full ac- 



count of the various electrometers at present known 

 under the article ELECTRICITY, with the results of ob- 

 swvationa made by some of the raoxt delicate of these 

 instruments. The difficulty of construction, as well as 

 the want of a determinate scale by which to measure 

 the intensity of atmospherical electricity, presents nn 

 obstacle to the general use of the tlcctronu-ter, which 

 we are afraid will not soon be surmounted, though the 



m we stc informed by .Mr Pbrfcir, m the frnt ftnm who applied the power of e< ipontic 

 psssMHr. md w. daaeform. s uslassilii mi mm of the Thumtmetrkat Hfgtometr, Mr. 



TVM. Vol. V.p.87, note. EB. 



ThafBMOKter, invented by Count Knnifbnl sod 



