WATER IN SOILS 11 



6. Light. The intensity of the light was estimated three 

 times daily in each plot until a ratio was established for each 

 plot. This ratio varies somewhat, being relatively greater 

 indoors on cloudy than on sunny days. The method used in 

 estimating light is the photographic method. Strips of 

 solio paper pasted together in a ribbon are placed on rollers 

 in an instrument invented by the author. The ribbon may 

 be exposed any number of seconds by means of a shutter in 

 the bux which contains it, and then rolled back in the dark 

 until needed for making the estimation. Solio paper if ex- 

 posed in full sunlight at noon during a series of increasing 

 periods of time as 2, 3, 4, 5 seconds, will print a series of 

 colors changing from a light pink for the shortest exposures 

 to a rich red brown for an exposure of 15 seconds. Such a 

 series of colors printed at meridian noon is taken for a daily 

 or local standard, and the colors obtained by time exposures 

 in each locality were matched with those of the standard. 

 Prom this the per cent of light at the time of the exposure 

 was estimated. The daily standard of sunlight varies dur- 

 ing the year with the sine of the angle of the sun's elevation. 

 Some day of the year must be taken as a fixed standard. 

 June 22 being the time of maximum light is taken in this 

 treatise as the fixed standard, and the light at meridian noon 

 for that day is called 100. In figuring light intensities by 

 this method, it was necessary to estimate each by the 

 light standard of June 22, in order to compare the light of 

 different days in different plots with each other. Thib was 

 done as follows : Suppose observations of light are taken 

 February 20, morning, noon, and evening. The noon expos- 

 ure is one of full sunlight, and its per cent of the fixed stand- 

 ard, June 22, is calculated as follows: Prom the astronomi- 

 cal tables of elevations for Lincoln prepared by Professor 

 Swezey, the angles of elevations are as follows : 



June 22, angle 72.6° 



Peb. 20, angle 38.4° 

 Turning to trigonometric tables for natural sines the 

 sines of these angles are : 



For 72.6" .9542 

 38.4° .6211 



