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 1° 
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 tke 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 represetitation 
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, when 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 
