622 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



threads will become dry and highly charged with electricity, 

 which causes the fibres to stand out from the threads and thus ren- 

 ders them liable to be broken. 



It is believed that, with slight experience, with this instrument 

 as a guide, any person with ordinary intelligence may maintain a 

 healthy, pure, and genial atmosphere within his dwelling; or, 

 using it out of doors, may ascertain the comparative salubrity of 

 different locations, and predict changes in the weather. 



The hygrodeik indicates the state of the atmosphere in relation, 



1st. To its actual temperature as indicated by the ordinary ther- 

 mometer (or the dry bulb thermometer of this instrument.) 



2d. Sensible temperature, or the temperature due to evapora- 

 tion (indicated by the wet bulb thermometer of this instrument.) 



The fact that the temperature due to evaporation is often quite 

 different (sometimes amounting to fifteen degrees), from the tem- 

 perature as indicated by the thermometer, is one of very great 

 importance, and one that is very 'commonly overlooked. 



When we cover a thermometer bulb with a thin piece of cloth, 

 and wet that cloth, we have an instrument which is sensible to the 

 temperature of the air in precisely the same degree that our lungs 

 are : that is, a person may feel too warm, or too cold, in a room 

 the temperature of which (as indicated by the common thermome- 

 ter) is 70 degrees; for the same reason that, in such a room, the 

 wet Inilb thermometer may indicate a temperature of 70 degrees, 

 or of only 55 degrees; the first of which, 70 degrees, is too warm, 

 while the latter, 55 degrees, is much too cold. 



The amount of difference between the dry and wet bull) ther- 

 mometers depends directly upon the amount of moisture in the 

 air. If the air contains all the moisture that it is capable of hold- 

 ing, as often occurs in wash and bathing rooms, it will be found 

 that there is no difference in the readings of the two thermome- 

 ters; but if the air is very dry, as we often find it to be in artifi- 

 cially warmed apartments, the readings of the two thermometers 

 may be very different. 



3d. The relative amount of moisture in the air. 



Air absorbes and holds in suspension water}' vapor in the same 

 manner 'that a sponge will hold water; but the amount that a 

 given bulk of air will hold depends upon its temperature. Thus 

 one cubic foot of air, at 32 degrees, will hold in suspension but 

 two grains of water, while one cubic foot of air, at . 6S degrees, 

 will hold seven and a half e:rains. When air at 32 degrees has 



