REMAINING LAWS OF NATURE. 157 



things which are to be compared, does necessarily con- 

 stitute the ultimate appeal, wherever such application 

 is practicable. But, in most cases, it is not practi- 

 cable : the objects cannot be brought so closely toge- 

 ther that the feeling of their resemblance (at least a 

 complete feeling of it) directly arises in the mind, 

 We can only compare each of them with some third 

 object capable of being transported from one to the 

 other. And besides, even when the objects can be 

 brought into immediate juxtaposition, their resem- 

 blance or difference is. but imperfectly known to us 

 unless we have compared them minutely, part by part. 

 Until this has been done, things in reality very dissi- 

 milar often appear undistinguishably alike. Two 

 lines of very unequal length will appear about equal 

 when lying in different directions ; but place them 

 parallel, with their farther extremities even, and if you 

 look at the nearer extremities, their inequality becomes 

 a- matter of direct perception. 



To ascertain whether, and in what, two pheno- 

 mena resemble or differ, is not always, therefore, so 

 easy a thing as it might at first appear. When the 

 two cannot be brought into juxtaposition, or not so 

 that the observer is able to compare their several 

 parts in detail, he must employ the indirect means of 

 reasoning and general propositions. When we can- 

 not bring two straight lines together, to determine 

 whether they are equal, we do it by the physical aid 

 of a foot rule applied first to one and then to the 

 other, and the logical aid of the general proposition 

 or formula, " Things which are equal to the same 

 thing are equal to one another." The comparison of 

 two things through the intervention of a third thing, 

 when their direct comparison is impossible, is the 

 appropriate scientific process for ascertaining resem- 



