Three functions, BsV, BsV2, and Bs2V where Bs is the southward component of the IMF and V, the solar wind speed, have been tested for their ability to predict geomagnetic activity as measured by the AL index. The data were selected from a period within 1 month of the June solstice for the years 1967--1974 inclusive, when AL apears to be most nearly a linear measure of magnetic activity. After correcting the index for processes not similar to substorms, separate comparisons of &Sgr;BmsVn with &Sgr;AL were made during isolated magnetic intervals so that the flux eroded from the forward magnetosphere could return, thus avoiding time delay estimates. Intervals during which there was evidence that the westward electrojet was displaced above or below the auroral observatory chain were eliminated. Under these carefully specified conditions, regression studies between AL and each of the three functions yield correlation coefficients of 0.97 for BsV2, 0.92 for BsV, and 0.82 for B2sV. A study of the variation of each of three functions of the type &Sgr;AL/&Sgr;BmsVn with Bs and with V has indicated that m?0.85 and n = 2. Some suggestions are offered to account for the diverse forms of the empirical prediction function used by various investigators.