=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 contour 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 exclusion of single curves. Combinations are made simply by tracing the curves to be compared upon the same sheet, it being understood that dissimilar curves, e. g., level and station, can not be combined. Colored inks are an absolute necessity in combining; the primary principle underlying their use is that curves that approach closely or cross each other must be traced in inks that contrast sharply. As elsewhere stated, it has been made the invariable rule to use the same color for the same level or point. This applies especially to temperature, but holds also for humidity, light, wind, and water-content, so that the color always indicates the level. For the same reason, it is applied to a combination of point curves for one station, though it is inapplicable to a series of point curves when these lie in the same level. Light readings above 6 feet and water-content readings below 15 inches necessitate the use of additional colors.