The psychrometric constant &psgr;, defined as the ratio ofA(corrected for radiation) to theApredicted by the convection theory, is shown to be a function of velocity. At infinite velocity, the new equation reduces to that of August (&psgr;=1); at zero velocity, to the result obtained by Maxwell, &psgr;=K/D.An improved method of obtaining the true wet‐bulb temperature is given, together with values of &psgr;, accurate to about 1 percent, for toluene, chlorbenzene and xylene. Straight lines are obtained when &psgr; for the latter two liquids against &psgr; for toluene at the same velocity. The theoretical psychrometric line for water is found; it is shown from theory that the line uncorrected for radiation possesses a stationary minimum value at velocities between 1000 and 10,000 ft./min., the radiation error just balancing the deviation from the August theory. In this rangeAis constant at 630×10−6, while at lower velocities it varies rapidly because of the change in the radiation error. For accurately reproducible results, the air velocity must be maintained at 1000 ft./min. or more.