506 Means of ascertaining the Degree of Humidity 



Art. II. On the Means of ascertaining the Degree qf Humidity most 

 suitable for the Atmosphere of Hot-houses. By George Wailes. 



(In a Letter to Mr. D. Beaton. Published with the consent of Mr. Wailes.) 



I am very desirous that some observations should be made, in 

 a few of the best collections in the kingdom, on the degree 

 of humidity of the atmosphere of our hot-houses, from which 

 some general law on the subject may be deduced. I am 

 convinced that the successful cultivation of plants depends as 

 much on that as on temperature. The collection under your 

 charge is, I am aware, as varied and as celebrated as most 

 collections ; and, judging from your communications to the Gar- 

 dener's Magazine that your wish is to combine science with 

 practice, I shall be glad if you will assist in the object I have 

 in view. No doubt, the chief reason why this branch of 

 horticultural meteorology has been so little attended to is, 

 the time and trouble required to make the observations by the 

 ordinary mode. We go into a stove, for instance, look at the 

 thermometer, and, if the degree of heat indicated is near the 

 point fixed upon as the mean, we are satisfied, and think 

 nothing of the trouble ; but, were the delicate manipulation re- 

 quired to ascertain the dew point by Daniel's hygrometer to 

 be gone through every time, I much fear the temperature, like 

 the humidity, would too often be regulated by the state of our 

 feelings at the time. I need hardly tell you that the dew point 

 is that degree of temperature, in any place, at which moisture 

 is deposited from the surrounding atmosphere upon any object 

 of that particular temperature, and that it depends, of course, 

 upon the humidity of the air. If, therefore, the air is very 

 moist, the slightest depression of the heat of the body in it 

 will cause dew to form ; and, on the contrary, if very dry, it 

 will require a considerable fall of temperature to produce that 

 result. Hence it is, that the cold produced by evaporation of a 

 liquid will be proportioned to the hygrometric state of the sur- 

 rounding medium ; and, by measuring that degree of cold, we 

 readily ascertain the degree of humidity. The common ther- 

 mometer is the best instrument for the purpose of showing the 

 temperature ; and we have only to keep its bulb wet with some 

 evaporating liquid of the same temperature as the medium it 

 is suspended in, to measure the degree of cold produced by 

 such evaporation, and thereby to find the dew point. To 

 facilitate the process, I have calculated and got printed the table 

 of which I send you a copy [printed on the next page], which 

 may be mounted on a card board, and suspended near the 

 thermometers. Of course, two thermometers are requisite, 

 one with a dry bulb to mark the temperature, the other with a 

 wet one to indicate the cold produced. 



