The changes in the amount of light due to the altitude of the sun are produced by the earth’s atmosphere. The absorption of light rays is greatest near the horizon, where their pathway through the atmosphere is longest, and it is least at the zenith. The absorption, and, consequently, the relative intensity of sunlight, can be determined at a given place for each hour of any sunshiny day by the use of chart 13. This chart has been constructed for Lincoln, and will serve for all places within a few degrees of the 40th parallel. The curves which show the altitude of the sun at the various times of the day and the year have been constructed by measurements upon the celestial globe. Each interval between the horizontal lines represents 2 degrees of the sun’s altitude. The vertical lines indicate time before or after the apparent noon, the intervals corresponding to 10 minutes. If the relative intensity at Lincoln on March 12 at 3:00 P.M. is desired, the apparent noon for this day must first be determined. A glance at the table shows that the sun crosses the meridian on this day at 9 minutes 53 seconds past noon at the 90th meridian. The apparent noon at Lincoln is found by adding 26 minutes 49 seconds, the difference in time between Lincoln and a point on the 90th meridian. When the sun is fast, the proper number of minutes is taken from 26 minutes 49 seconds. The apparent noon on March 12 is thus found to fall at 12:37 P.M., and 3:00 P.M. is 2 hours and 23 minutes later. The sun’s altitude is accordingly 36°. If the intensity of the light which reaches the earth’s surface when the sun is at zenith is taken as 1, the table of the sun’s altitudes gives the intensity at 3:00 P.M. on March 12 as .85. For places with a latitude differing by several degrees from that of Lincoln, it is necessary to construct a new table of altitude curves from the celestial globe. It is quite possible to make a close approximation of this from the table given, since the maximum and minimum meridional altitude, and hence the corresponding light intensity, can be obtained as indicated above. For Minnehaha, which is on the 105th meridian, and for other places on standard meridians, i. e., 60°, 75°, 90°, and 120° W., the table of apparent noon indicates the number of minutes to be added to 12 noon, standard time, when the sun is slow, and to be subtracted when the sun is fast. The time at a place east or west of a standard meridian is respectively faster or slower than the latter. The exact difference in minutes is obtained from the difference in longitude by the equation, 15° = 1 hour. Thus, Lincoln, 96° 42′ W. is 6° 42′ west of the standard meridian of 90°; it is consequently 26 minutes 49 seconds slower, and this time must always be added to the apparent noon as determined from the chart. At a place east of a standard meridian, the time difference is, of course, subtracted.