96 THE HABITAT 



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

 interval, and the extreme range of the curve or combination. For example, 

 in the case of temperature, it is most convenient to assign a value of i 

 Centigrade to each centimeter, since the thermometers used read to one- 

 fifth of a degree, which corresponds exactly to the 2-millimeter units of each 

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

 and the actual limits must be determined for the particular results to be 

 employed. For the same region, it is very desirable that the unit interval 

 and the range be the same, in order that all curve sheets may admit of direct 

 comparison. Indeed, it is greatly to be hoped that in the future ecologists 

 will agree to a uniform system of curve-plotting, cartography, etc., as 

 the geographers are beginning to do in the construction of maps. The 

 major intervals are written, or, better, typewritten, at both sides of the 

 sheet, and the time or space intervals are indicated at the top. Each curve 

 sheet is properly labeled, and essential data indicated. The readings are 

 taken from the field record, and their proper positions indicated by a dot. 

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

 rather than flowing; and the line, after having been carefully checked, is 

 traced in ink. 



140. Kinds of curves. Curves are named both with reference to the 

 factor concerned and the position or sequence of the readings. The 

 factors which lend themselves most readily to this method of representation 

 are the variable ones, water-content, humidity, light, temperature, and 

 wind, and corresponding curves are distinguished. Altitude and slope may 

 likewise be shown by means of curves, but the use of cross section or con- 

 tour lines serves the same purpose and is more natural. With regard to 

 time and position, curves are distinguished as level, station, and point 

 curves. A level curve is one based upon readings made at the same level 

 through a series of stations or of habitats, e. g., the level curve of surface 

 temperature. The station curve represents the various levels or points at 

 which readings are made in a single station. The point curve has for a 

 basis the hourly or daily variation of a factor at a particular point or level 

 in a station. All of these may be simple curves, Vvdien established upon a 

 single reading for a series, or mean curves when they are based upon the 

 mean of a number of readings. Curves which show the extremes of a 

 factor, i. e., the maximum and minimum, are also extremely valuable, 

 though a combination of the two for comparison is preferable. 



141. Combinations of curves are invaluable for bringing similar curves 

 together, and permitting ready comparison of them. For this, and also 

 because they save space, they are regularly employed to the almost complete 



