64 BULLETIN OF THE NEW YORK STATE MUSEUM 



temperature becomes stationary, and while a more intense 

 heat increases the rapidity of evaporation it can not raise 

 the temperature, therefore the quicker we can supply the 

 amount of heat required for the evaporation of a given 

 quantity of water, the shorter must be the time in which it 

 can be accomplished. A given quantity of water, exposed 

 in two vessels, in one of which it is but 1 inch high and in 

 the other 10 inches, to the same amount of heat on their 

 heating surfaces, will evaporate in different spaces of time. 

 The water in the vessel with the larger heating surface 

 evaporates in one-tenth of the time of that in the other 

 vessel, since the former has ten times more heating surface, 

 and therefore transfers the heat ten times faster, hence we 

 may say ; that the quantity of water that can be evaporated 

 in a given time and at a given temperature is directly pro- 

 portional to the area of the heating surface. 



We assumed before that we supplied both vessels with 

 the same amount of heat, could we raise the temperature 

 under the vessel with the smaller heating surface, so as to 

 impart to the water it contains as much heat as to our large 

 vessel in exactly the same time, the result would be the 

 same. Two vessels with equal heating surfaces and contain- 

 ing equal quantities of water, will show unequal results if 

 one be heated more strongly than the other, the one more 

 strongly heated evaporating more water in the same given 

 time. With equal heating surfaces the amount of water 

 evaporated in a given time is directly proportional to the 

 temperature of the heated surfaces. The temperature at 

 which water boils under ordinary circumstances at sea 

 level is 212 Fahrenheit, but at high elevations where the 

 air is lighter it boils at a lower temperature, and in deep 

 mines at a higher temperature caused by the increased 

 pressure of the air, hence it follows that the boiling tem- 

 perature of water is dependent on the pressure of the air 

 resting on the boiling liquid. 



The fact is well known that water in an open vessel un- 



