Another alternative consists in having two different key-numbers. In the report the words are indicated by figures always changing, first in the case of the one, and then of the other key-number. In this way one and the same word, indicated twice in the report, may each time be indicated differently. For instance, the word "division" is number 765 in the word-book; the key-numbers are 5,000 and 6,125. The first time the word "division" is indicated by the number 4,235 (that is, 5,000-- 765) and the second time by 5,360 (that is, 6,125--765). The main disadvantage of all these various means of communication lies in that a book is essential, from which the deciphering will have to be done. The book in question is easily lost, or is not at hand at the required moment, and then the cipher is not only useless, but the work caused in obtaining the message is wasted. An instance of this occurred in 1870, when one of the German generals received a message which he could not decipher at once, since the dictionary which the headquarter staff had arranged with him as a code-book was in a wagon which had been left behind. The same misfortune befell the commander of a French territorial division at Chalons-sur-Marne, who could not decipher a telegram, since by an oversight he had sent his cipher with the archives to Chateau Thierry. For the ciphering of messages by spies the above systems are also disadvantageous because most spies can carry no books with them on their missions. Thus writes Klembovski, who made a study of espionage on active service, but was unable to arrive at any method of transmitting messages which should be free of marked disadvantages. As to his contentions regarding the use of ciphers, it has been stated--though on what authority is hard to ascertain--that the German secret service will decipher any message in any language in a given space of time, no matter what code may be used. In one of Edgar Allan Poe's stories there is a means shown of deciphering practically any code in which the characters are constant-- that is, in which the same sign represents the same letter each time it is used--and it is quite probable that one used to the deciphering of code-messages could work out any code. For a code must be built up on some system, and therefore some reversal of the system must exist by means of which the message can be deciphered without the aid of a key.