Chapter II · THE HABITAT (continued 3)
Reading edition
142. The amplitude of all the curves described above is determined by the unit values of the factors concerned, while the length is dependent upon the number of stations, points, or times. The value assigned the latter upon the plotting paper is purely arbitrary, but it is most convenient to fix this at the centimeter square. The unit value for temperature is 1° Centigrade per square, each subdivision of the latter representing 0.2, and the range being 22 degrees. For water-content curves, each square represents a value of 2 per cent, the smaller square being 0.4 per cent, and the range 2–48 per cent. The unit value for humidity is taken as 5 per cent, making each small square 1 per cent, and giving room on the sheet for the entire range from 1–100 per cent. Owing to the anemometer used, curves of wind velocity have been based upon the number of feet per minute. One hundred feet is taken as the unit value, and the range is from 0–2200 feet. The unit value for the curve of light intensity is .005. Each small square is .001, which permits a range from .001 to .01 on one sheet. Consequently, when it is desired to plot the curve of a series of habitats with a range in intensity greater than this it is necessary to use a double sheet. This is the usual device when the range of curves is too great, except where the excess is slight. In this case the curve is left open at the top, and the value which the crest attains is indicated. All curves in combination are labeled at the beginning or left to indicate the level, station, or point, and at the end or right to show the time, or day, if this is not the basis of the curve or series.
The discussion that precedes deals exclusively with curves representing factors determined in the field. It applies with equal force to results obtained by instruments in control houses. In these, however, all factors except those directly experimented with, usually water-content and light, are practically equalized, and the curves based upon them are used chiefly to show how nearly equal they have become. The important curves are those of the water-content series, both holard and chresard, and of the shade tents. Where several houses are differentiated with respect to temperature or humidity, curve series of both these factors are necessary.
Factor Means and Sums
143. It has been shown elsewhere that the daily mean of temperature can be closely approximated from the maximum and minimum of both day and night. Maximum-minimum instruments for the other factors are lacking, however, and for light, humidity, and wind these values can only be obtained 99from the ecograph which makes it possible to get the exact mean from the sum of all the hour readings. When it comes to the seasonal mean, the ecograph is even more necessary, exception being made for water-content, in which case a number of readings on various days through the season will suffice. The value of factor means for diagnosis and for curves has already been sufficiently commented upon, and the feasibility of factor sums already indicated.
100