124 THE PRINCIPLES OF SCIENCE. 



qualitative reasoning, the syllogism was only by analogy 

 or simile called an Instrument. Swift satirically described 

 the Professors of Laputa as in possession of a thinking 

 machine, and in 1851 Mr. Alfred Smee actually proposed 

 the construction of a Relational machine and a Differential 

 machine, the first of which would be a mechanical dic- 

 tionary and the second a mode of comparing ideas ; but 

 with these exceptions I have not yet met with so much 

 as a suggestion of a reasoning machine. It may be added 

 that Mr. Smee's designs, though highly ingenious, appear 

 impracticable, and in any case do not attempt the per- 

 formance of logical inference 1 ". 



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 Phi- 

 losophical Transactions k , and it would be both tedious 

 and 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- 



' See his work called ' The Process of Thought adapted to Words and 

 Language, together with a description of the Relational and Differential 

 Machines.' Also 'Philosophical Transactions/ [1870] vol. 160, p. 518. 



k ' Philosophical Transactions,' [1870] vol. 160, p. 497. 'Proceedings 

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

 T- 343- 



