96 THE PRINCIPLES OF SCIENCE. [CHAP. 



labour required is the only impediment, when once the 

 meaning of the premises is rendered clear. But the 

 amount of labour is often found to be considerable. The 

 mere writing down of sixty-four combinations of six 

 letters each is no small task, and, if we had a problem of 

 five premises, each of the sixty-four combinations would 

 have to be examined in connection with each premise. 

 The requisite comparison is often of a very tedious 

 character, and considerable chance of error intervenes. 



I have given much attention, therefore, to lessening both 

 the manual and mental labour of the process, and I shall 

 describe several devices which may be adopted for saving 

 trouble and risk of mistake. 



In the first place, as the same sets of combinations occur 

 over and over again in different problems, we may avoid 

 the labour of writing them out by having the sets of 

 letters ready printed upon small sheets of writing-paper. 

 It has also been suggested by a correspondent that, if any 

 one series of combinations were marked upon the margin 

 of a sheet of paper, and a slit cut between each pair of 

 combinations, it would be easy to fold down any particular 

 combination, and thus strike it out of view. The com- 

 binations consistent with the premises would then remain 

 in a broken series. This method answers sufficiently well 

 for occasional use. 



A more convenient mode, however, is to have the series 

 of letters shown on p. 94, engraved upon a common school 

 writing slate, of such a size, that the letters may occupy 

 only about a third of the space on the left hand side of 

 the slate. The conditions of the problem can then be 

 written down on the unoccupied part of the slate, and the 

 proper series of combinations being chosen, the contra- 

 dictory combinations can be struck out with the pencil. 

 I have used a slate of this kind, which I call a Logical 

 Slate, for more than twelve years, and it has saved me 

 much trouble. It is hardly possible to apply this 

 process to problems of more than six terms, owing to 

 the large number of combinations which would require 

 examination. 



