Atmosphere in Houses. 367 



alluded to in the pages of your Magazine, which is a record of 

 so many valuable inventions. 



It proceeds on the fact, ascertained long ago, that if a piece 

 of glass one degree colder than the atmosphere be surrounded 

 with air saturated with moisture, dew will be deposited upon it. 

 All that is necessary is to provide a plain tumbler, the thinner 

 in the glass the better, and a jug of water a few degrees colder 

 than the air of the house ; cold spring water should be best. 

 Pour the water into the tumbler, the glass will immediately be 

 cooled down below the air of the house ; and dew will be depo- 

 sited on the sides of the tumbler, till the heat of the water rises 

 up to the point to which the air of the house is saturated. To 

 ascertain this point, continue to pour the water from the tumbler 

 into the jug, wiping the tumbler carefully on the outside with a 

 dry towel, and again pouring the cold water back into the tumbler, 

 watching carefully the moment that dew ceases to be deposited 

 on the outside of the tumbler ; and a thermometer, plunged into 

 the water precisely at that period, will denote the point to 

 which the air of the house is saturated. Unless where great 

 accuracy is required, it may perhaps be sufficient to wipe the 

 outside of the tumbler, and continue to do so so longas moisture 

 is deposited. To pour the water in and out from the jug and 

 tumbler is more troublesome, but more correct. The thinner 

 the tumbler, and the more alkaline the glass, the more quickly 

 will dew be deposited, and the more perfect the result. The 

 thermometer in the house will tell the temperature of the air; and 

 the difference between it and the temperature of the water, at 

 the moment dew ceases to be deposited on the glass tumbler, 

 will point out how many degrees the moisture in the air is below 

 the point of saturation. 



The quantity of vapour which can exist in the air at either of 

 the above-mentioned degrees, or any other between the freezing 

 and boiling point, may be known by consulting the table annex- 

 ed ; which shows the elasticity of vapour, as ascertained by the 

 experiments of Mr. Dalton, and which, though slightly differed 

 from by others, will be sufficiently accurate for all ordinary pur- 

 poses. Water, and many other liquids, have a tendency to 

 assume the form of vapour at all temperatures ; but the quantity 

 is regulated by the degree of heat which keeps the particles of 

 vapour asunder, and gives them elasticity. The quantity of 

 vapour which can exist in the atmosphere is therefore regulated 

 by the elasticity of that vapour. The mean height of the baro- 

 meter at the sea shore in this country is 30 in. nearly, or 29 - 82. 

 As shown by the annexed table, at 32° (the freezing point), the 

 vapour of water is capable only of supporting a column of mer- 

 cury 0*2 in. in height ; consequently, this being only the 5th part 

 of an inch, the air at the freezing point is only capable of con- 



