There being no obvious line of research, he began a laborious and detailed investigation into the possibilities of substitution, that is, one of those ciphers in which a number or other sign is used to denote a letter. It was clear that single numbers were insufficient for this purpose, as in that case only ten letters of the alphabet could be used. Some combination was therefore involved, and French tried various schemes of addition to meet the case. But though he got three men to assist him in the details of his various tests, he could not find anything which gave the least suggestion of an intelligible combination. While engaged in this manner, he noticed that so far as the pounds were concerned there were no less than three similar pairs, numbers 2 and 22, 3 and 15, and 10 and 21. He examined these pairs for some time, and then he suddenly made a discovery which seemed to show that at last he was on the right track. He had put the figures down beside each other, so: £ _s._ _d._ No. 2 568 5 0 No. 22 568 2 3 when suddenly he noticed that if the shilling and pence of each item were added the result would be the same: 5+0=5; 2+3=5. Eagerly he turned to the other pairs and wrote them out similarly, £ _s._ _d._ No. 3 1039 1 3 No. 15 1039 0 4 and, £ _s._ _d._ No. 10 748 3 9 No. 21 748 5 7 Here he saw at a glance that the same thing obtained, the pounds alone, and the pence and shillings added together, making two similar pairs, and therefore presumably standing for the same word. This discovery restored all his eager interest. It seemed definitely to prove three things, each several one of which afforded him the liveliest satisfaction. First, these combinations of figures proved that there really was some underlying scheme, and that in its turn involved the hidden message; secondly, they showed that he, French, was on the direct road towards a solution; and thirdly, they indicated a code or cipher built up of pairs of numbers, a frequent combination, embracing many well-known varieties of cryptogram. His next step was, therefore, to rewrite the list in dual column, the pounds in front, the pence and shillings added together behind. This gave him a new jumping-off place in the following: 328—6 568—5 1039—4 936—9 713—11, and so on.