108 THE PRINCIPLES OF SCIENCE. [CHAP 



The Logical Abacus soon suggested the notion of a 

 Logical Machine, which, after two unsuccessful attempts, 

 I succeeded in constructing in a comparatively simple and 

 effective form. The details of the Logical Machine have 

 been fully described by the aid of plates in the Philo- 

 sophical Transactions, 1 and it would be needless to repeat 

 the account of the somewhat intricate movements of the 

 machine in this place. 



The general appearance of the machine is shown in a 

 plate facing the title-page of this volume. It somewhat 

 resembles a very small upright piano or organ, and has a 

 keyboard containing- twenty-one keys. These keys are of 

 two kinds, sixteen of them representing the terms or 

 letters A, a, B, b, C, c, D, d, which have so often been 

 employed in our logical notation. When letters occur on 

 the left-hand side of a proposition, formerly called the 

 subject, each is represented by a key on the left-hand half 

 of the keyboard ; but when they occur on the right-hand 

 side, or as it used to be called the predicate of the pro- 

 position, the letter-keys on the right-hand side of the 

 keyboard are the proper representatives. The five other 

 keys may be called operation keys, to distinguish them 

 from 'the letter or term keys. They stand for the stops, 

 copula, and disjunctive conjunctions of a proposition. 

 The middle key of all is the copula, to be pressed when 

 the verb is or the sign = is met. The key to the extreme 

 right-hand is called the Full Stop, because it should be 

 pressed when a proposition is completed, in fact in the 

 proper place of the full stop. The key to the extreme 

 left-hand is used to terminate an argument or to restore 

 the machine to its initial condition ; it is called the Finis 

 key. The last keys but one on the right and left com- 

 plete the whole series, and represent the conjunction or in 

 its unexclusive meaning, or the sign -I- which I have 

 employed, according as it occurs in the right or left hand 

 side of the proposition. The whole keyboard is arranged 

 as shown on the next page 



ential Machines. Also Philosophical Transactions, [1870] vol. 160, 

 p. 518. 



1 Philosophical Transactions [1870], vol. 1 60, p. 497. Proceedings 

 of the Royal Society, vol. xviii. p. 166, Jan. 20 1870. Nature, vol. i, 

 P- -343- 



