26 



The National Geographic Magazine 



and at present there is no way to avoid 

 this labor and expense. The physical ex- 

 ertion of climbing towers, carrying a bas- 

 ket of instruments along, making read- 

 ings all day, was not inconsiderable, as it 

 took from forty minutes to one hour to 

 do the work on a single tower. There 

 were five towers to attend to, and this 

 had to be repeated seven times every day. 

 We had the good fortune to incur no ac- 

 cidents of a personal nature, though sev- 

 eral thermometers were broken in the 

 operations. 



The wind during the afternoon and 

 evening often reached 40 miles an hour, 

 and even 50 miles per hour, and on the 

 top of the towers it was no little task to 

 guard the delicate instruments against 

 injury in the violent gusts. The Sierra 

 Nevada mountains, 10,000 to 12,000 feet 

 high, poured down into the Reno Valley 

 a powerful current of cool air every day 

 with wonderful regularity, and this 

 caused the strong winds to prevail. 



INTERESTING RESULTS 



We succeeded in keeping up the cur- 

 rent reductions of the observations to an 

 advanced stage, and so gained an idea of 

 our results before leaving Reno, the 18th 

 of September. They were such as to 

 show clearly enough the source of the 

 discrepancies that have been mentioned. 

 For example, it was soon seen that the 

 evaporation from the pans on the top of 

 the towers was from two to three times 

 as much as at the foot of the towers, in 

 or near the water, and that there is a 

 regular progression from pan to pan. 

 On the other hand, in the dry field where 

 tower Xo. 1 was located there is no such 

 important difference, the evaporation 

 being practically the same all the way up 

 the tower. At tower No. 5, in the alfalfa 

 field, where the ground was wet from 

 irrigation, we found that the retardation 

 of evaporation was confined to 10 or 15 

 feet from the ground. 



It became clear enough that the reser- 

 voir, which is about 1,000 feet long, 

 covers itself with a sheet of invisible 

 vapor about 30 feet thick, and that this 



vapor acts like a blanket upon the fresh 

 evaporation rising from the water. Dur- 

 ing the process of evaporation there are 

 two principal stages : First, the water 

 must turn into vapor, and the amount 

 differs according to the temperature. 

 Thus, for o° centigrade 1 cubic centi- 

 meter becomes 211,000 cubic centimeters 

 of vapor ; at ioo° centigrade it becomes 

 1,658 cubic centimeters of vapor. Second, 

 these columns of fresh vapor must stream 

 off into the air by diffusion and mixture, 

 and the capacity of the air to receive it 

 depends upon its own temperature and 

 dew-point, which determine its vapor con- 

 tents. If the air is dry and the difference 

 between the dry-air temperature and 

 dew-point is great, there will be rapid 

 evaporation, but if small the evaporation 

 will be slow. The wind is an important 

 factor, because it brings new masses of 

 air, not so much saturated, over the 

 water surface, and thus keeps the super- 

 posed air more ready to receive the newly 

 evaporated vapor. There are at least 

 five factors to take into the account : 



1. The diffusion factor — a function 

 of the height above the surface of the 

 water. 



2. The temperature of the water and 

 its capacity to deliver vapor — a function 

 of its vapor pressure. 



3. The capacity of the air to receive 

 vapor — a function of the difference be- 

 tween the dry air temperature and the 

 dew-point. 



4. The velocity of the wind, the func- 

 tion being the square of the velocity. 



5. The wind coefficient, being a func- 

 tion of the height above the ground. 



A small pan in the open air, away from 

 a sheet of water, evaporates faster than 

 the same pan in like conditions located 

 within the blanket of vapor lying over 

 a lake, because these factors operate to- 

 gether differently. Hence it is evident 

 that the location of the experimental pan 

 relative to the water surface of a lake 

 must be fully taken into the account. The 

 many pans at Reno gave differing re- 

 sults, grading up and down the towers, 

 and from the center of the reservoir to 

 the distant dry fields, in such a way as 



