# Page:Popular Science Monthly Volume 80.djvu/441

1. Einstein's actual equation is ${\displaystyle \scriptstyle D^{2}=4/3\ .\ E/K\ .\ t}$, in which ${\displaystyle D^{2}}$ is a quantity obtained by squaring each individual displacement and then taking the mean of these squares, E is the mean kinetic energy of agitation of the drop, K is a resistance factor depending upon both the medium and the drop, and t is the length of the time interval used. If the average displacement D is used instead of the average square of the displacements ${\displaystyle D^{2}}$ the correct form of the equation is
${\displaystyle \scriptstyle D={\sqrt {\frac {8}{3\pi }}}{\frac {E}{K}}t}$.