Note 42 — Chapter XXI · POLYBIUS.

I have not ventured to interrupt my narrative with an account of the invention as it was described by Polybius in more than one conversation, but I will give it here for the benefit of such readers as may be interested in the subject. The plan which Polybius seems to have found in use was a very curious one, and, it is evident, far from being effective. The two bodies of men which would have to communicate by signal were provided with two vessels of exactly the same diameter and depth, and with outlets for the water of exactly the same size. Divisions were marked on them, and each division was appropriated to some common contingency in military affairs, as for instance, "Cavalry has arrived", "Cavalry is wanted", "Food is short", &c. The party desirous to communicate showed a torch. The other replied in the same way to indicate that they were attending. Another torch was shown by the first party. This meant that the water had been set flowing. The other replied in the same way, and set the water flowing in their vessel. When the desired point had been reached a third signal was shown. As soon as this signal was seen, the other side observed how far the water in their vessel had sunk. The defect was that only a few out of the innumerable contingencies of war could be thus communicated. The system perfected by Polybius was much more effective. The alphabet was divided into five groups of five letters each. The party wishing to communicate, which I will in future speak of as No. 1, called the attention of the other (No. 2) by raising two torches, and this signal was acknowledged in the same way. No. 1 then showed one, two, three, four, or five torches on the left to indicate which group he was about to use, and then one, two, three, four, or five on the right to indicate the letter in the group. An observing-glass with two tubes was necessary for No. 2 to enable him to distinguish between right and left. I will give an example, taking it, for convenience, from our own alphabet. "Cavalry wanted" is the message which No. 1 desires to send. The groups of letters would be—

1. a b c d e. 2. f g h i j. 3. k l m n o. 4. p q r s t. 5. u v w x y.

z might be neglected, as practically of no use. (In the Greek alphabet of 24 letters the fifth group would be one letter short. This, of course, would not matter.)

C is shown by 1 left and 3 on right; a by 1 and 1; v by 5 and 2; l by 3 and 2; r by 4 and 3; y by 5 and 5.

And similarly with "wanted".