26 A COMPARISON BETWEEN THE CLIMATES OF 



This table is constructed from the indications of Daniell's hygro- 

 meter ; an instrument consisting of a glass tube about six or eight 

 inches long, bent twice at right angles, and terminated, at each ex- 

 tremity, in a bulb. One of the bulbs, which is usually coloured, 

 contains a very delicate little thermometer and a small quantity of 

 ether ; the thermometer dips into the ether, which may be driven, 

 by the heat of the hand, into either of the bulbs. Upon cooling 

 the empty one, which may be done by pouring a few drops of ether 

 upon it, the other immediately becomes cooled also, from the eva- 

 poration of the fluid inclosed within it, while the thermometer, 

 dipping into it, shews how much it is cooled. When using the in- 

 strument, you narrowly watch the coloured bulb while the cooling 

 process is going on ; and at the moment when it becomes wetted 

 with dew, you note the degree at which the thermometer included 

 in it stands, and this is the dew point. Sometimes a few drops of 

 ether are sufficient to produce the effect, and the inclosed thermo- 

 meter falls perhaps only a degree or two ; this shews that the dew 

 point is hardly below the temperature of the air, and the atmos- 

 phere is damp. At other times it is required to wet the bulb seve- 

 ral times ; and dew is not produced upon the coloured ball until 

 the inclosed thermometer falls ten, fifteen, or twenty degrees below 

 the temperature of the air ; the atmosphere, under these circum- 

 stances, being very dry. 



This instrument is usually fitted to a little brass pillar, having 

 affixed to it another small thermometer, which shews the tempera- 

 ture of the air ; so that the comparison between this and the dew 

 point may be made at the same moment. 



That the object and use of the hygrometer may be thoroughly 

 understood, it should be remembered that the atmosphere consists 

 of two essentially distinct fluids, one consisting of permanently 

 elastic gases — constituting the air, properly so called — the other of 

 aqueous vapour, which, within the range of the temperature of the 

 atmosphere, is capable of assuming the aeriform, the fluid, and the 

 solid state. It is upon this that all the most important meteorolo- 

 gical phenomena depend, such as dew, fog, cloud, rain, hail, or 

 snow. It is to discover the existing quantity of this aqueous vapour 

 when in its aeriform or invisible shape, relatively to the tempera- 

 ture of the air, that Daniell's hygrometer is made use of. 



The dew point frequently is as high as the temperature of the 

 air, during heavy rain, in damp weather, in the evening, and at 

 night. The dew point is very often much below the temperature 

 of the air in clear fine weather, and especially with N. E. winds ; 



