When a despatch or communication runs great risk of falling into the hands of an enemy, it is necessary that its contents should be so veiled, that the possession of the document may afford him no information whatever. Julius Cæsar and Augustus used cyphers, but they were of the utmost simplicity, as they consisted merely in placing D in the place of A; E in that of B, and so on; or else in writing B for A, C for B, etc. Secret characters were used at the Council of Nicæa; and Rabanus Maurus, Abbot of Fulda and Archbishop of Mayence in the ninth century, has left us an example of two cyphers, the key to which was discovered by the Benedictines. It is only a wonder that any one could have failed to unravel them at the first glance. This is a specimen of the first: .Nc.p.t v:rs:·:s B::n.f:c.. :rch. gl::r.::s.q:.:: m:rt.r.s The secret of this is that the vowels have been suppressed and their places filled by dots,--one for i, two for a, three for e, four for o, and five for u. In the second example, the same sentence would run--Knckpkt vfrsxs Bpnkfbckk, etc., the vowel-places being filled by the consonants--b, f, k, p, x. By changing every letter in the alphabet, we make a vast improvement on this last; thus, for instance, supplying the place of a with z, b with x, c with v, and so on. This is the system employed by an advertiser in a provincial paper which I took up the other day in the waiting-room of a station, where it had been left by a farmer. As I had some minutes to spare, before the train was due, I spent them in deciphering the following: Jp Sjddjzb rza rzdd ci sijmr, Bziw rzdd xr ndzt: and in ten minutes I read: “If William can call or write, Mary will be glad.” A correspondence was carried on in the _Times_ during May 1862 in cypher. I give it along with the explanation. Wws.--Zy Efpdolj T dpye l wpeepc ez mjcyp qzc jzf--xlj T daply qfwwj zy lww xleepcd le esp tyepcgtph? Te xlj oz rzzo. Ecfde ez xj wzgp--T lx xtdpclmwp. Hspy xlj T rz ez Nlyepcmfcj tq zywj ez wzzv le jzf.--May 8. This means--“On Tuesday I sent a letter to Byrne for you. May I speak fully on all matters at the interview? It may do good. Trust to my love. I am miserable. When may I go to Canterbury if only to look at you?” A couple of days later Byrne advertises, slightly varying the cypher: Wws.--Sxhrdktg hdbtewxcv “Tmwxqxixdc axzt” udg pcdewtg psktgexhtbtce ... QNGCT. “Discover something _Exhibition-like_ for another advertisement. Byrne.” This gentleman is rather mysterious: I must leave my readers to conjecture what he means by “Exhibition-like.” On Wednesday came two advertisements, one from the lady--one from the lover. WWS. herself seems rather sensible-- Tydeplo zq rztyr ez nlyepcmfcj, T estyv jzf slo xfns mpeepc delj le szxp lyo xtyo jzfc mfdtypdd.--WWS., May 10. “Instead of going to Canterbury, I think you had much better stay at home and mind your business.”