shct. ii.] DISSERTATION SECOND. ** 



spar, or the want of if, may arise from the greater or less 

 compression under which the fusion of the basalt was per- 

 formed. This has served to explain a great difficulty in 

 the history of the mineral kingdom. 



Comparative anatomy is full of analogies of this kind, 

 which are most instructive, and useful guides to discovery. 

 It was by remarking in the blood-vessels a contrivance si- 

 milar to the valves used in hydraulick engines, for prevent- 

 ing the counter current of a fluid, that Harvey was led to 

 the discovery of the circulation of the blood. The analo- 

 gies between natural and artificial productions are always 

 highly deserving of notice. 



The facts of this class, however, unless the analogy be 

 very close, are apt to mislead, by representing accidental 

 regularity as if it were constant. Of this we have an ex- 

 ample in the supposed analogy between the colours in the 

 prismatick spectrum and the divisions of the monochord. 

 The colours in the prismatick spectrum do not occupy the 

 same proportion of it in all cases : the analogy depends on 

 the particular kind of glass, not on any thing that is com- 

 mon to all refraction. The tendency of man to find more 

 order in things than there actually exists, is here to be 

 cautiously watched over. 



VII. 31 ono dick, or singular facts, are the next in order. 

 They comprehend the instances which are particularly 

 distinguished from all those of the genius or species to 

 which they belong. Such is the sun among the stars, the 

 magnet among stones, mercury among metals, boiling foun- 

 tains among springs, the elephant among quadrupeds. So 

 also among the planets, saturn is singular from his ring, the 

 new 'planets are so likewise from their small size, from be- 

 ing extrazodiacal, &c. 



Connected with these are the irregular and deviating in- 

 stances, in which nature seems to depart from her ordinary 



10 



