Atmospheric Moisture to Hot-houses. 283 



be subtracted by condensation alone from a saturated atmosphere 

 35| gallons of water. 



It is true that a saturated atmosphere is not constantly main- 

 tained, nor is to be desired in forcing-houses, but it will be ob- 

 served that in this estimate no allowance is made for escape of 

 heated air, which would probably more than compensate for the 

 difference between a saturated atmosphere, and that which ought 

 to be maintained in a forcing-house. In the cultivation of Or- 

 chideae, the atmosphere must be kept at a state much more nearly 

 approaching to saturation, so that for this purpose the foregoing 

 estimate of waste by condensation will probably not exceed the 

 truth. 



It may, however, be possible to obtain a somewhat more ac- 

 curate calculation of the amount of condensation which will take 

 place, supposing the atmosphere of a forcing-house, as above 

 described, to be maintained at a moderate and natural degree of 

 moisture. In the absence of more exact data, we may fairly 

 take the mean degree of dryness of April, May, and June, the 

 growing months, as indicative of the mean degree of dryness 

 which ought to exist in a forcing-house. And in the absence of 

 observation, which might enable us to deduce a hygrometric mean, 

 we may take as an approximation the mean dryness, as observed 

 daily at 9 a.m. at the Royal Society's apartments in London. 



It may be observed, that during these months the variation 

 between dryness and dampness during the twenty-four hours is 

 at its utmost. The atmosphere being supersaturated during 

 eight or nine hours of night, and exhibiting various degrees of 

 dryness during the day. The dryness at 9 a. m. is undoubtedly 

 greater than the mean dryness of the twenty-four hours, but it 

 may be fairly assumed to represent a state of atmosphere whole- 

 some to vegetation. It appears that the mean dryness, i. e. the 

 difference between the temperature and dew point for April, May, 

 and June, 1838, was as follows : — April, mean at 9 A. M. 6° 8' ; 

 May, '8° 5'; June, 5° 1': giving a mean of 6° 8', say 7°. Now, 

 if a mean degree of moisture, corresponding with this, be kept 

 up in the forcing-house above mentioned, the difference between 

 the dew point of the house and the temperature of outward air 

 will be 23° ; therefore, the quantity of moisture condensed per 

 minute will be the difference between that which will saturate 

 437 cubic feet of air at 5S°, and that which will saturate the 

 same volume of air at 35°, the supposed temperature of outward 

 air, when the stove is 65°. Now, this quantity is 1289'15 grains, 

 rather more than two thirds the amount which would be con- 

 densed from a saturated atmosphere, or about one gallon per hour. 

 Having ascertained, or at least obtained, some approximate 

 estimate of the waste of moisture which takes place from the at- 

 mosphere of our hot-houses, it remains to devise the best method 



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