18 
HINTS TO TRAVELLERS. 
and if the temperature were to fall ever so little there would be more 
water-vapour present in the air than it could hold and some would 
separate out and condense into dew or rain, hence the temperature of 
saturation is called the dew-point. But if air saturated at 50° is warmed 
up say to 60° it can then contain more water-vapour than it has, and the 
temperature would require to fall 10° before dew or rain could form. 
When the air is not saturated water exposed to it evaporates rapidly 
until the maximum quantity of water-vapour is again present, a larger 
quantity corresponding to the higher temperature. At any given tem¬ 
perature the essential thing to know about the humidity of the air is 
the additional amount of water-vapour it could take up before becoming 
saturated, or in other words the humidity relative to the maximum 
humidity possible at the existing temperature. The relative humidity is 
expressed in percentages of the maximum humidity possible (saturation) 
at the actual temperature of observation. It may be measured by two 
methods, (1) finding the dew-point or temperature at which the amount 
of vapour present saturates the air; (2)'by finding the rate at which the 
air allows evaporation to proceed; the farther the air is from saturation 
the more rapid is the rate. 
The dew-point may be found directly by means of an instrument by 
which the air is cooled down until it begins to deposit moisture on a 
polished surface, but such an instrument is inconvenient to handle when 
travelling. It may also be found indirectly by calculation from the 
relative humidity. 
The relative humidity is most easily calculated from the rate of 
evaporation. It is one of the laws of evaporation that heat is required 
to change liquid into a vapour, and when evaporation is going on heat 
is being abstracted from surrounding bodies, and they are growing 
colder. By allowing evaporation to take place from the bulb of a 
thermometer the rate of evaporation may be measured by the fall of 
temperature produced, and tables have been constructed to convert 
the differences between the wet and dry bulb readings into relative 
humidities. 
The wet-bulb thermometer consists of an ordinary thermometer, the 
bulb of which is covered with clean muslin and kept moist by means of 
a piece of cotton lamp-wick dipping into a small vessel of pure water. 
Care must be taken to have the water quite pure and free from salt. 
