In practice an accurate comparison of time at different places is made, not by transporting chronometers, but by electromagnetic signals---usually wireless time-signals for places on the earth, and light-signals for places in the solar system or stellar universe. Take two clocks at $A$ and~$B$, respectively. Let a signal leave~$A$ at clock-time~$t_{1}$, reach~$B$ at time~$t_{B}$ by the clock at~$B$, and be reflected to reach $A$ again at time~$t_{2}$. The observer~$S'$, who is at rest relatively to the clocks, will conclude that the instant~$t_{B}$ at~$B$ was simultaneous with the instant $\frac{1}{2}(t_{1} + t_{2})$ at~$A$, because he assumes that the forward velocity of light is equal to the backward velocity. But for~$S$ the two clocks are moving with velocity~$u$; therefore he calculates that the outward journey will occupy a time $x/(c - u)$ and the homeward journey a time $x/(c + u)$. Now \begin{align*} \frac{x}{c - u} &= \frac{x(c + u)}{c^{2} - u^{2}} = \frac{\beta^{2}x}{c^{2}}(c + u),\displaybreak[0] \\ \frac{x}{c + u} &= \frac{x(c - u)}{c^{2} - u^{2}} = \frac{\beta^{2}x}{c^{2}}(c + u). \end{align*} Thus the instant~$t_{B}$ of arrival at~$B$ must be taken as $\beta^{2}xu/c^{2}$ later than the half-way instant $\frac{1}{2}(t_{1} + t_{2})$. This correction applied by~$S$, but not by~$S'$, agrees with~\Eq{(11.4)} when we remember that owing to the FitzGerald contraction $x = x'/\beta$. \PageSep{29} Thus the same difference in the reckoning of simultaneity by $S$ and~$S'$ appears whether we use the method of transport of clocks or of light-signals. In either case a convention is introduced as to the reckoning of time-differences \index{Time!convention in reckoning}% at different places; this convention takes in the two methods the alternative forms--- (1) A clock moved with infinitesimal velocity from one place to another continues to read the correct time at its new station, \emph{or} (2) The forward velocity of light along any line is equal to the backward velocity\footnotemark.\footnotetext {The chief case in which we require for practical purposes an accurate convention as to the reckoning of time at places distant from the earth, is in calculating the elements and mean places of planets and comets. In these computations the velocity of light in any direction is taken to be $300,000$~km.\ per~sec., an assumption which rests on the convention~(2). All experimental methods of measuring the velocity of light determine only an average to-and-fro velocity.}