INTRODUCTION. 11 



Notwithstanding many diffuse treatises which relate to them we cannot 

 boast of having reduced them to any determinate laws ; and yet there are 

 some meteorological facts which well deserve our attention. Natural history 

 is the last of the sciences that it will be necessary for us to notice. Some 

 may think it superfluous to attempt to give so superficial a sketch of this 

 most extensive subject as our plan will allow ; but it is still possible to 

 select some general observations respecting the methods of classification, as 

 well as the philosophy of natural history, which, although very concise, 

 may yet be in some measure instructive. This third division of the course 

 would properly include, together with the general properties of matter and 

 the particular actions of its particles, the whole science of chemistry ; but 

 the variety and importance of chemical researches demand a separate and 

 minute discussion ; and the novelty and beauty of many of the experi- 

 ments with which the labours of our cotemporaries have presented us, and 

 which will be exhibited in the theatre of the Royal Institution by the pro- 

 fessor of Chemistry, are sufficient to make this department of natural phi- 

 losophy the most entertaining of all the sciences. 



Such is the whole outline of our plan, and such are the practical uses to 

 which the arts and sciences comprehended in it are principally applicable. 

 Before we proceed to the examination of its several parts, we must pause to 

 consider the mode of reasoning which is the most generally to be adopted. It 

 depends on the axiom which has always been essentially concerned in 

 every improvement of natural philosophy, but which has been more and 

 more employed, ever since the revival of letters, under the name induction, 

 and which has been sufficiently discussed by modern metaphysicians. 

 That like causes produce like effects, or that in similar circumstances 

 similar consequences ensue, is the most general and most important law of 

 nature ; it is the foundation of all analogical reasoning, and is collected 

 from constant experience by an indispensable and unavoidable propensity 

 of the human mind. 



It does not appear that we can have any other accurate conception of causa- 

 tion, or of the connexion of a cause with its effect, than a strong impression 

 of the observation, from uniform experience, that the one has constantly 

 followed the other. We do not know the intimate nature of the connexion 

 by which gravity causes a stone to fall, or how the string of a bow urges 

 the arrow forwards ; nor is there any original absurdity in supposing it 

 possible that the stone might have remained suspended in the air, or that 

 the bowstring might have passed through the arrow as light passes through 

 glass. But it is obvious that we cannot help concluding the stone's weight 

 to be the cause of its fall, and that every heavy body will fall unless sup- 

 ported ; and the pressure of the string to be the cause of the arrow's mo- 

 tion ; and that if we shoot, the arrow will fly ; if we hesitated to make 

 these conclusions, we should often pay dear for our scepticism. This ex- 

 planation is sufficient to show the identity of the two expressions, that like 

 causes produce like effects, and that in similar circumstances similar con- 

 sequences ensue. And such is the ground of argument from experience, 

 the simplest principle of reasoning after pure mathematical truths, which 

 appear to be so far prior to experience, as their contradiction always im- 

 plies an absurdity repugnant to the imagination. 



