Henry Ernest Dudeney/Modern Puzzles/173 - Difference Squares

by : $173$

 * Difference Squares
 * Can you rearrange the nine digits in the square so that in all the eight directions the difference between one of the digits and the sum of the remaining two shall always be the same?


 * $\begin{array}{|c|c|c|}

\hline 4 & 3 & 2 \\ \hline 7 & 1 & 9 \\ \hline 6 & 5 & 8 \\ \hline \end{array}$


 * In the example shown it will be found that all the rows and columns give the difference $3$:
 * (thus $4 + 2 - 3$, and $1 + 9 - 7$, and $6 + 5 - 8$, etc.),
 * but the two diagonals are wrong, because $8 - \paren {4 + 1}$ and $6 - \paren {1 + 2}$ is not allowed:
 * the sum of the two must not be taken from the single digit, but the single digit from the sum.
 * How many solutions are there?