222 Journal of Agricultural Research voi.x, no.s 



This same ratio holds true for the rim of metal projecting above the 

 ground, mentioned previously. The concentration or radiation effect of 

 this strip of metal is 1 2 times as great for the small tank as it is for the 

 large one, in proportion to the exposed area of the water. Higher tem- 

 peratures during the average day and lower temperatures during the 

 average night are reached by the small tank than by the large one. 

 The temperature means are nearly the same, that for the large tank 

 being slightly greater. However, the effect of the higher day tempera- 

 tures may possibly be greater than that of the lower night temperatures 

 and make the net evaporation due to temperature effect alone greater 

 from the small tank than from the larger. Results from a tank exposed 

 on all sides (No. 8) having a mean temperature lower, a night tempera- 

 ture lower, and a much higher day temperature than No. 7, are consist- 

 ent with the foregoing statement. 



There may be an appreciable vapor blanket effect on evaporation tanks 

 of the size experimented with. An air movement great enough to remove 

 such a covering from a tank i foot in diameter would not so quickly 

 change the air above a pan or tank 12 feet in diameter. To pass over 

 the 1 2-foot diameter in the exact time that is required to pass over the 

 I -foot diameter would necessitate a wind velocity 12 times as great 

 over the 12-foot tank as over the i-foot tank, and this variation can 

 not occur. The records of the actual wind velocity for the year show 

 a low velocity of 11.9 miles in 12 hours, or approximately 1.5 feet per 

 second. This occurred once. Eighteen 12-hour periods are recorded 

 having a wind movement of 20 miles or less in 12 hours. The average 

 for the year is 59.5 miles in 12 hours, or approximately 7.5 feet per 

 second. At that rate the largest tank would have a complete change 

 of air covering at intervals of 1.6 seconds. Whether or not this effect 

 is appreciable in the amount of evaporation from tanks of the size used 

 is as yet a matter of opinion. If it is appreciable, then the result would 

 be that of increasing the evaporation from the small tank over that 

 from the large one. 



But one previous investigation which would give information as to 

 the ratio sought in the research being described has been made. That 

 was in connection with the extensive study made at Salton Sea for the 

 determination of an evaporation law. In order to compare the results 

 of that with those found at Denver the equipment is described (3, p. 



1 134): 



In order to test the ratio of evaporation from pans of different sizes, our records 

 include the following combinations: (i) A 4-foot pan self-registered hourly and a 

 2-foot pan along side on the ground near Tower No. i ; (2)3 ow of 3 pans, 2-foot, 4-foot, 

 6-foot in diameter, on a platform on Tower No. 3, about half a mile from shore, and as 

 near the water as was practicable; (3) a row of 4 pans, 2-foot, 4-foot, 6-foot, 12-foot on 

 a series of adjoining rafts floating in the Salt Creek slue in calm water. The ratios are 



