THE WATER OF THE HABITAT 35 



of ways in which curves can be combined is almost endless. For 

 the beginner, however, the most satisfactory are those in which 

 but one factor is taken into account. The most useful are those 

 in which the curves of temperature, or of any other factor, for 

 the different levels are drawn together. Similarly, the curves 

 showing the variations of temperature for each station may be 

 combined. A combination of the greatest value is obtained by 

 contrasting the curves of holard and chresard for a series of sta- 

 tions. A very interesting combination may be obtained l)y 

 arranging the series of curves for two distinct habitats, such as 

 prairie and forest, side by side upon the same sheet, thus per- 

 mitting the direct comparison of the curves for various factors. 



48. Plotting curves. The plotting-paper employed is ruled 

 in centimeter squares which are divided into 2-millimeter units. 

 The sheet is 24x18 cm. in size, thus making it possible to file 

 the curves in the record book. A fine-pointed pen, such as the 

 Spencerian No. 1, is used for plotting. The inks used are the 

 Higgins Waterproof Inks, which are made in the following colors: 

 black, violet, indigo, blue, green, yellow, orange, brown, brick 

 red, carmine, and scarlet. In addition to being waterproof, they 

 make it possible to combine curves readily without destroying 

 their identity. It is also a great advantage to use the same color 

 invariably for the same kind of curve. 



In plotting a curve, it is first necessary to fix the value of 

 the centimeter square or interval, as well as the extreme range 

 of the curve itself. For example, in the case of temperature, a 

 value of 1 Centigrade is assigned to each centimeter, since the 

 thermometers used read to one-fifth of a degree, which thus cor- 

 responds with the value of the 2-millimeter units of each square. 

 The length of the sheet permits a range of 22 degrees Centigrade, 

 within which the greater number of temperature curves for a 

 particular season will fall. It is very desirable that the unit 

 interval and the range be the same for each factor, in order that 

 all curve sheets for the same region may admit of direct com- 

 parison. The major intervals are indicated at both sides of the 

 sheet, and the time or the space intervals at the top. The roatl- 

 ings upon which the curve is based are taken from the field record, 

 and the proper position of each is indicated by a dot. The dots 

 are first connected by a pencil line, the curves being made angular 

 rather than flowing. After being carefully checked, the line is 



