II YG ROME TRY. 



597 



Iff. 





again l>y the process of breathing, so charged with 

 moisture that the point of deposition is 85, as we have 

 determined by experiment. Hence each cubic inch must 

 carry off, at an average, .007231 2 grains of moisture ; 

 but as a cubic inch of the air inhaled contained .00263 1 5 

 gnum,theactual loss of moisture by breathing is.0045997 

 grains fur every cubic inch of air expelled. Now, a 

 man usually makes about 12 inspirations in a minute; 

 and since each inspiration requires about 25 cubic 

 inches, he should thus lose by the process of breathing, 

 in the circumstances we have supposed, nearly eight 

 cubic inches of water daily. Again, at the height 

 of 15,000 feet above the level of the sea, a cubic 

 inch of air, in its mean hygrometric state, contains, 

 according to the Table, only .0002287 grains of mois- 

 ture ; and therefore each expiration must carry off. 1 7506 

 grains of water from the body. But if we suppose the 

 number of inspirations to be inversely as the density of 

 t!ic air, a person in that elevated situation should make 

 about 18 inspirations in a minute ; and therefore lose 

 by breathing about ,' r grains of moisture in the same 

 time, or nearly 18 cubic inches of water daily. The 

 quantity of moisture lost by breathing being more than 

 doable in the latter case of what it is in the former, 

 accounts in a very satisfactory manner for the great 

 thirst and parched state of the fauces, which all tra- 

 vellers have experienced at considerable heights in the 

 atmosphere. 



Cnwof On !/"* We *hall conclude these general remarks on the 

 "2J* h- hygrnroetric state of the atmosphere, by observing, 

 that since the dryness of the air depends upon the ex- 

 CCM of its temperature above the point of deposition, 

 in whiter, when the temperature of caves, cellars, &c. 

 is greater than that of the air, these subterraneous pla- 

 ces will always seem dnr ; whereas in summer, w hen 

 their temperature is below the mean temperature of 

 the ah-, the contrary should be found to hold, and they 

 ought to appear damp. In this country, the daily 

 mean minimum temperature of the season will be obser- 

 ved to coincide with the mean temperature of these si- 

 tuations about the middle of April, and to exceed it from 

 that time till the middle of September. Hence caves, 

 mines, fie. which have a free communication with the 

 ir, ought to be damp from April to September, and 

 dry from September to April. The same remark is 

 perhaps applicable to places situated near the sea, par- 

 tin.: irly in high latitudes. 



99. The preceding investigations having conducted 

 us to an accurate and precise knowledge of the consti- 

 tution of the atmosphere with respect to humidity, we 

 shall finish the subject of hygrometry with a concise 

 view of the quantity of moisture which, under the va- 

 rious forms of dew, rain, hail, and snow, descends, 

 during the annual round of the seasons, to the surface 

 of the earth, in different latitudes. The principal and 

 perhaps the only cause which occasions a precipitation 

 of moisture, by making it pass from the vaporous to the 

 liquid state, must be change of temperature ; and since 

 we already know the mean quantity of moisture which 

 exists in the atmospherical columns for the different lati- 

 tudes, it would not be difficult to determine what por- 

 tion of it ought to be precipitated at some particular 

 place, provided we also knew the extent of the change 

 of temperature to which the superincumbent atmosphere 

 was exposed. But though we can ascertain with toler- 

 able accuracy the mean annual temperature of a place, in 

 terms of the latitude, it would seem to defy the powers 

 5 



Qnutityof 



of calculation to anticipate, with sufficient precision, the Hygrome 

 temperature which the different strata of the atmosphere "T- 

 immediately over it will possess on some particular day ; ^"" "Y"^ 

 and therefore, all we can do in the present case, is, to 

 endeavour to form an estimate on general principles, and 

 calculate the mean annual quantity of rain, for differ- 

 ent situations, on the supposition that in the course ef 

 a year corresponding changes of temperature take place 

 throughout the whole mass of the atmosphere. As these 

 changes will give rise to depositions of moisture, which 

 must be more or less copious according to the mean 

 quantity of vapour existing in the atmospherical co- 

 lumns over any place, it will be easy to determine, on 

 the principles we have laid down, and with the assist- 

 ance of the Tables in $ 51 and .94, the mean quantity 

 of rain for the different latitudes. For, since by the 

 Table in 51, the mean annual evaporation is 36.2 

 inches, and the mean quantity of moisture in the at- 

 mosphere at once 4. 1 inches, for each of the atmosphe- 

 rical columns having a square inch for its base, it ap- 

 pears that the quantity of moisture evaporated annual- 

 ly is 8 times as great as the atmosphere retains in the 

 state of vapour at a time. Hence if the numbers which 

 express in inches the heights of the aqueous columns, 

 equal to the columns of vapour existing in the atmo- 

 sphere, for the mean temperature of the latitudes, as 

 stated in the Table in 94, be multiplied by 8.8, the 

 products will give the mean annual quantity of rain, 

 also in inches, for the several latitudes, at the interval 

 of.V. We thus obtain the numbers in the following 

 Table, which will be found to coincide pretty accurate- 

 ly with the mean quantity of rain which falls in differ- 

 ent places, as determined by observation. 



100. The limits prescribed for this article will not 

 allow us to follow out this branch of the subject in de- 

 tail ; but we shall resume the consideration of it under 

 the head of MITEOROLOOY. In the mean time we may 

 remark, that the change of temperature by which the 

 deposition of moisture from the atmosphere is produced, 

 will be of four descriptions : 1st, The annual change of 

 temperature ; 2d, The daily change of temperature ; 

 3d, The change of temperature occasioned by the trans- 

 ference of the aerial columns from one parallel of lati- 

 tude to another ; and 4th, The change of temperature 

 arising from the elevation or depression of the atmo- 

 spherical strata, by the inequalities of the earth's sur- 

 face. If all these circumstances be duly taken into ac- 

 count, they will explain, in the most satisfactory man- 

 ner, every anomaly respecting the quantity of rain for 

 places under the same parallel of latitude. (A) 



