In this paper it is shown that the value of the similarity exponent &agr; derived analytically by Fujimoto and Mishkin [J. Fluid Mech.89, 61 (1978); Phys. Fluids21, 1933 (1978)] is exactly the same as that found by Stanyukovich [Unsteady Motion of Continuous Media, (Academic, New York, 1960)]. Since the result found by Stanyukovich is an approximation to &agr;, Fujimoto and Mishkin’s claim to have an exact expression of &agr; is false. The two methods are outlined and Stanyukovich’s result is simplified to show its equivalence to the work of Fujimoto and Mishkin.