HYGROMETRY. 



581 



( 1+. 002086 t S*\ /447.4+A 



l +.002086* 32/ = 9r V447.4+V = 



48. Apain, since the number of grains contained in a 

 cubic inch of vapour at the temperature T, and under a 



.0068544 <p r 

 of S J inchesof mercury is 



Ijmmt- fifth part of a degree of Fahrenheit. Let the tempera- 

 ture thus obtained be represented by r, and let ? r be 



"V ' the corresponding elastic force of vapour at its maxi- 

 mum of solution ; alo let/' be the elastic force of the 

 vapour, in the state of attenuation in which it actually 

 exists in the atmosphere, at the temperature t ; then 

 since nothing more is necessary but to dilate this vapour 

 from the temperature T to the temperature I, in order 

 to reduce it from the force Q to the force/', 



* - 



M," * 



Of !-.-.. 

 the -^ II 



"^T.T' l """" ! " " " " "-"-* " 1 +.002088 7^32' 



rvo- / / _ T \ 



_ t fat. and since/' = 0J 1 + 4474 i T h tbtnten ifg' be 



t, the number of grains of moisture in a cubic inch of air, 



the temperature of which is /, and the point of deposi- 

 tion T, and actual tension^ ', 



,_ .0068544/' _ 3.2859 /' 

 = 1 + .008086 1 ~ 447.4+ I 



And if the preworeof the barometer be /I, 



_ -}09MJ 

 ~ 447.4 + I ' 



.1095J,if> T 



To illustrate this formula by an example, let us sup- 

 pose that when the temperature of the air is 65, the 

 point of deposition 51*, and the height of the barome- 

 ter S9.5, it were required to find, in these circumstan- 

 ce*, the number of grains of moisture in a cubic inch of 

 mir, 



-U* [ 9 *r_.l0953 X89.5 X .42779 _ (X)g7$fr7 



1 4*7. + * 4r7.4 + 54 



49. Another method of determining the value of/', 

 and the corresponding value of f', may be derived from 



the formula of Mr Dalton, E = *^-{ /,f ) in wn ich 



E denote* the n urn tier of grains, evaporated in a mi- 

 nute, from the nurtac* of water contained in a cylin- 

 drical vessel, 6 inche* in diameter, and 1 inch deep ; 

 f t the entire tension of vapour at the temperature of the 



air I; and / ' the tension which it actually exert*. 

 The coefficient m is different, in different circum- 

 stances: when the air is perfectly calm and tranquil, 

 it i* 120; in a moderate brcexe, it amounts to about 

 150; and in a high wind it rise* to upwards of 180. 

 If we employ the mean 150, we obtain 



or E= 5 (//-/') 



severs! important problems in meteorology. Thus Ilygremc- 

 \vhen the mean temperature and annual evaporation try ' -. 



p 



of a place are known, the formula /': // ^ will 







enable us to determine the mean state of the air of that Mean point 

 place, with regard to moisture. Let us take for ex- ofdeposi- 

 ample the mean annual evaporation of Great Britain, ** on lor 

 which is reckoned about 24 inches ; this in a minute P at Bri " 

 would be -00004563 inches, and applied to a cylindrical a 

 vessel 6 inches in diameter, would correspond to -32587 

 grains. 



-32587 

 Hence/' =// g 



And if we reckon the mean temperature 50, and sub- 

 stitute for_/J the corresponding elastic force of vapour, 

 taken from the table in 31, we obtain, 



Toese formula? may be applied to the solution of 



/' = -37345 = -308276 



O 



Having found/ the number of grains correspond- 

 ing to that tension, at the temp. 50, may be found I>v 

 the formula for determining g in 38 : we thus have 



. 



It will be found by the table that this number of 

 grains corresponds to 44M, which, in the present case, 

 is the mean temperature at which moisture will begin 

 to deposit itself in Great Britain ; that is, about 6 be- 

 low the mean temperature, * 



50. The value of g 1 might have been found without 



3*2859 f 



having recourse to the formula g ---- , by ta- 



447 '4 +/ 



king a proportional part of the weight of moisture, 

 held in the vaporous state, at the entire tension of 50", 

 corresponding to the actual tension -308276. Thus the 

 entire tension at 50, being -37345 inches, and the 

 weight of the corresponding quantity of moisture, in 

 the vaporous state, -00246714 grains, we hare 



pain. 



OOJ46714 



37345 : -308876 



00203652 



If we take the mean annual evaporation of a place j frui . 

 near the equator, the mean point of deposition is found ' ot faJ,! 

 to be nearly the same number of degrees below the tion for a 

 mean temperature, as we have determined it in the place near 

 caw of Great Britain. Thus at Whydah, on the coast he e 1"- 

 of Africa, the mean temperature of which by the for- t01 * 

 mula, <=58 + 27 cos. 2 LaL is 84-2, the mean annual 

 evaporation is stated at 64 inches, or at the rate of 

 86899 grain* per minute from the disc of Mr Dalton ; 

 and hence 



*=/ =/ =i H2T9 - 



By proceeding as before, the number of grains corre- 

 sponding to a cubic inch of vapour, under the tension 

 .968992 inches, and at the temperature 84.2, will be 

 found to be .00598944, which correspond* to the quan- 

 tity of moisture, in a cubic inch of vapour, for the en- 

 tire tension of 78.6; so that, at Whydah, the mean 

 point of deposition is 5.6 below the mean temperature. 

 It seems probable, from these results, that ttie moan 

 point of deposition, for any place, is about G below the 

 mean temperature ; though we admit, observations are 



-,. 



Or DoUon found by obstmatioo, that the mean temperature of deposition, at l.irerpool, was T below tbt temperature- of that 



