M E T E E L G Y. 



163 



Metctfrolo- few cases where it has been used, care has not always 

 gy. been taken to give either the precise time of the oo- 

 y ' " J servation, or the corresponding temperature. The 

 consequence is, that we know very little regarding the 

 mean height of the instrument under any latitude, and 

 till les* of the true relative or absolute humidity of 

 the atmosphere, even when its indications have been 

 noted. To ascertain the true mean height, the self- 

 registering hygrometer described above is perhaps in- 

 dispensably necessary ; but if the instrument in its 

 simplest form has hitherto been so seldom employed, 

 it i> hardly to be expected that a more cemplex one will 

 soon be brought into general use. In the mean time, 

 To Bad the however, there is a method by which this desideratum 

 irae mean may, in part at least, be supplied. It has already been 

 Mghl o/ shewn, that the mean temperature at ten morning and 

 the hyfro. evening does not differ from the mean temperature of 

 met< - r - the whole day, and since the relative humidity of the 

 trmnphrrr depends not a little on temperature, it may 

 be inferred that the mean of the hygrometer at these 

 hours, will be an approximation to the mean of the 

 day. It is upon this principle that we have hitherto 

 registered the hygrometer, and we shall now state the 

 height of us) instrument, as well as of the ther- 

 ', Car each month of the Ian two years. 



should the temperature increase, the hygrometer will 

 also rise, and continue to do so indefinitely, with every 

 new accession of heat, though the actual quantity of 

 moisture remains nndiminished In the article referred 

 to, a formula is given, by which the indications of the 

 hygrometer, which are of themselves vague and with- 

 out any distinct meaning, may be converted into pre- 

 cise and definite expressions. The most important 

 facts which may be deduced from that formula, are the 

 three following. 



lit, The point of deposition. It is known from the 

 law which regulates the solution of water in the atmo- 

 sphere, that a portion of air holding any given quanti- 

 ty of moisture in a state of vapour, will, if cooled down, 

 arrive at a temperature where it will become saturated, 

 or incapable of dissolving more moisture, than it for- 

 merly contained, and consequently if its tem|>erature 

 be farther diminished, it will begin to deposit a por- 

 tion of that moisture in the form of dew or rain. The 

 temperature at which it begins to do so is called the 

 I'oint of Deposition. 



'2tt, The actual quantity of moisture in a given por- 

 tion of the atmosphere. This quantity is expressed by 

 the formula in decimals of a grain in one cubic inch"; 

 but as the fraction is small, we have expressed it in 

 our register, by decimals of a grain in 100 cubic 



Tmptrs. 

 tan 10 b 

 taktn at 



From the above, and other observations made in 

 1814, 1815, and 1 >!''. it appears that the roeaui height 

 of Leslie's hygrometer, situated 185 feet above the le- 

 vel of the sea, 20 miles from the coast, and under the 

 latitude 56 s 2,', U about 13.2. This fact, however, 

 conveys no definite idea respecting the actual humidity 

 of the atmosphere, nor can the indications of the in- 

 strument, taken by themselves, furnish us with any 

 correct information on this subject. It has been shewn, 

 under the article HYGKOMCTRY, that these indications 

 arc of no use, unless the state of the thermometer at 

 the time of observation be also known, Ix-c.ui- 

 found I'rotn actual experiment, that though the absolute 

 ' moisture in the atmosphere should remain 

 unchanged, the hygrometer will rise with an increase, 

 and fall with a diminution of temperature. When the 

 hygrometer stands at ero, or when the wet and dry 

 thermometers indicate the same temperature, it may be 

 red. whatever the tem|xT;iturc be, that the atmo- 

 sphere u completely saturated with moisture; but 



Meteorolo. 



Results of 

 Anderson* 



Point of 

 deposition, 



or com ' 



3d, The relative humidity of the atmosphere. It 

 was observed above, that in the hygrometer, zero is a 

 fixed point denoting complete saturation, but that the 

 scale extends upward* indefinitely, or in other M 

 there is no point in the scale which at all teiujx-r. 

 denotes absolute tlijifcass By the formula alluded to, 

 this defect is completely remedied. Absolute moisture 



ing denoted by 100, and absolute dryness by 0, it 



ay be determined what intermediate degree of mois- 

 ture is indicated by any given state of the hygrometer. 

 This is in fact converting an indefinite into" a definite 

 scale, and by employing 100 to denote extreme mois- 

 ture, all other quantities are expressed in hundredth!) 

 of what would produce complete saturation. These 

 three particulars may be illustrated by the following 

 example. 



Suppose that, at any given hour, the hygrometer 

 indicates SO, according to the millesimal scale, anil the 

 thermometer in the same circumstances 50; required 

 the jMiint of deposition, the actual quantity of mm-ture 

 in loo cubic inches of air, and the relative humidity. 



By applying the formula, it will be found that the 

 point of deposition is 40*.3 Fahrenheit, the ;ic iu.,1 

 quantity of moisture in 100 cubic inches of air, 0.178 

 of a grain, and the relative humidity 7 ! ; that in, the 

 atmosphere, in these circumstances, contains 73 hun* 

 dredths of the whole moisture which it is capable of 

 holding in solution at the temperature of 50, and which 

 amounts to .178 of a grain in 100 cubic inches; and if 

 the temperature were reduced to 40.3, the hygrome- 

 ter would sink at the same time to 0, after which the 

 moisture would begin to be deposited. 



Let MS suppose, now, that the hygrometer, as be- 

 fore, indicates 2 ', but that the temperature is 60. In 

 this case the point of deposition will be found to be 

 53".l, the actual quantity of moisture in 100 cubic 

 inches O.26J), and the relative humidity 80. Such are 

 the different degrees of moisture which may actually 

 exist in the atmosphere, while the hygrometer stand's 

 at the same point. 



We are aware that the lalxnir and difficulty of calcu- 

 lating the result of each single observation may bo 



Actual 

 quantity of 

 lnoi lur 



^ 



Reiatlr* 

 humidity 



"' 



mo *'' t ' r * t 



Example* 

 to iiiu.ir*t 

 And rwn* 



forlnul 



Appltr*. 

 lion <>l ih 

 formula 

 may b 

 fimpliBrd 



