The repeated solution of more or less involved equations by actual computation, with or without the use of tables, is being advantageously replaced in many fields by mechanical solution through the use of nomograms. In these devices the mathematical equation that defines the relationship existing between a set of variables is represented by a geometric pattern on a plane surface, one scale line in this pattern being constructed for each variable in the equation. The equation itself may then be solved for any one unknown by laying a straight-edge in appropriate positions across the pattern and thereby reading off the answer without any arithmetic calculation whatsoever. In the ecological field, pertinent application of this time-saving principle may be made for the conversion of basic psychrometric measurements of humidity into their various alternative forms of expression, or the conversion of any of these expressions into another form more suitable to a problem at hand. Two nomograms are presented herein to meet this need.' While they were designed primarily for the direct conversion of wet and dry bulb psychrometer readings into the corresponding vapor pressure, dew point, relative humidity, and vapor pressure deficit values, they may also be used for conversions within the latter group of alternative expressions. The nomogram in figure 1 is a graphic portrayal of the equations relating dry and wet bulb psychrometer temperatures, vapor pressure, dew point, and relative humidity for the barometric pressures of 30, 29, 27, and 25 inches. The use of