582 



HYGROMETRY. 



Hygrowe. still wanting to warrant so general a conclusion. Mr 



v ^'-^ Dalton affirms, that the point of deposition is generally 



r ~ Y " from 1 to 10 below the mean heat of the 24 hours; 



we have usually found it to be from 6 to 7. * 

 Mean an- 51. The following formula, which we have deduced 

 nual evapo- on the supposition that the mean point of deposition is 

 " n m 6 below the mean temperature, seems to agree pretty 

 ' well with observation. Let t be the mean temperature ; 

 f, the entire force of vapour corresponding to t ; and 

 <P T the entire force of vapour corresponding to the tem- 

 perature T, or f 6; then if A be the mean annual 

 evaporation in inches, 



. = 365 (f t 



447-4 



The mean daily evaporation, for any place, according 



to this formula, is f t <p M 4- 77 r __L or nearly 

 T \ 447.4 -f- T/ 



// VT. 



Let it be required to find, by it, the mean annual 

 evaporation, at a place in Lat. 45, the mean tempera- 

 ture of which is 57. 



-M 1 * 



447.4 + 



= 365 f 



annual evaporation corresponding to every 5 of differ- 

 ence of latitude. These appear to increase, according 

 to some regular law, from the pole to the parallel of 

 30 ; after which they begin to diminish, and continue 

 to decrease to the equator. The greatest differences 

 take place near the limits of the trade winds. Hence 

 it might be inferred, that the most copious depositions 

 of moisture should occur a few degrees on either side 

 of the tropics. 



52. As the temperature of every place, for the whole 

 year, ranges between two extreme points correspond- 

 ing to the alternations of summer and winter ; so it ex- 

 hibits every 24 hours, a corresponding difference with 

 respect to the vicissitudes of day and night. In the 

 case of the daily change of temperature there is some 

 interval between the maximum and the minimum, which 

 may be regarded as the temperature belonging to the 

 season of the year ; and, though this point will not al- 

 ways be a mean between these extremes, it will in ge. 

 neral approach very near it. If this mean temperature 

 were to rise and sink regularly, as the season advanced 

 and declined, without being subject to daily fluctuation, 

 the quantity of moisture which could exist in the at- 

 mosphere, at any given time, might be determined with 

 the utmost precision ; since nothing more would be ne- 

 cessary for that purpose, than to calculate the maximum, 

 quantity of vapour for the temperature, by the formula 

 in 39 A variety of causes, however, which are too 

 complicated in their nature to admit of being reduced 

 under any general law, (but of which the vicissitude 

 of day and night is the principal,) continually conspire 

 to raise and depress, by turns, the temperature of every 

 place above or below its mean level for the season ; and 

 hence the quantity of moisture in the atmosphere will 

 generally be less than the quantity corresponding to the 

 mean temperature, but at all times nearly equal to that 

 belonging to the minimum temperature. If it be less 

 than the latter quantity, the process of evaporation will 

 gradually supply the deficiency ; if it be greater, the 

 excess will quickly be precipitated in the form of dew, 

 rain, or snow, according to the temperature, and the 

 extent of its depression, below the minimum temper- 

 ature. 



In short, the mean point of deposition, which we for- 

 merly represented by T, and for which <p r is the corre- 

 sponding elastic force of vapour, must be nearly the 

 same as the minimum temperature of any place on any 

 given day. 



It appears by the following observations, extract- 

 ed from the meteorological journal for 1815 kept by 

 the Rev. Mr Gordon of Kinfauns, that the minimum 

 temperature of Perth, and consequently the mean point 

 of deposition for that place, is about 6 below the mean 

 temperature, thus coinciding very nearly with the re- 

 sult formerly deduced from theory, as the mean point 

 of deposition for Great Britain and the globe in gene, 

 ral. 



In the following Table, the depression of the minimum 

 temperature below the mean temperature for each month, 

 has evidently some relation to the indications of Lea. 

 lie's hygrometer. This relation will be better under- 

 stood, after we have explained more particularly the 

 manner in which that instrument is affected by the 

 elasticity of the vapour existing in the atmosphere ; at 

 present we shall only remark, that as observations made 

 with any hygrometer are of very little value, unless the 



. o " " "i>iii, in a general way, by the mean height of the clouds, and the rate of 



dimiuution of temperature as we ascend in the atmosphere. 



Hygrome- 

 try. 



The point 

 of deposi- 

 tion coin- 

 cides near- 

 ly with the 

 minimum 

 tempera- 

 ture for the 



inches* 



= 365 (.47328 .3864 x 1.012) = 30.02. 

 For the purposes of calculation, the expression may 

 be reduced to the approximated, but more commodi- 



(81 

 ft ; 9f_ 



By means of this formula, we have deduced the fol- 

 lowing Table, which expresses the mean annual and 

 daily evaporation, from the equator to either pole, for 

 the different parallels of latitude, at the interval of 5'. 

 The mean annual temperature corresponding to each 

 latitude, was derived from Meyer's formula. 



The last column contains the differences of the mean. 



