INTRODUCTION. 5 



moment we detect. 5 ' Hence we see that dynamics 

 is a science principally of calculation, chemistry of 

 experiment, and natural history of observation. 



These terms are sufficiently characteristic of the 

 methods employed in cultivating the three branches 

 of natural science, and also serve to point out the 

 different degrees of certainty attainable in each. 

 Calculation, as it were, commands nature, and de- 

 termines her phenomena more accurately than ob- 

 servation can make them known. Experiment forces 

 her to unveil, and observation watches her when 

 refractory, and is always on the alert to surprise and 

 detect her. 



There is also a principle peculiar to natural his- 

 tory, which is employed with considerable advan- 

 tage in a variety of instances, in the prosecution of 

 that study. It is that of the conditions of existence, 

 commonly termed final causes. As no material body 

 can exist, unless it combine all the conditions which 

 render such existence possible, its component parts 

 must be so arranged as to admit of this possibility, 

 not only in itself but in relation also to whatever sur- 

 rounds it. The analyses of these conditions, or 

 final causes, frequently lead to the discovery of 

 general laws, demonstrated as clearly as are those 

 derived from calculation or experiment: and it is 

 not until all the laws of general physics, and those 

 also which are brought to light by these conditions 

 of existence are exhausted, that the natural histo- 

 rian is reduced to simple observation. 



Mere observation will, however, avail but little 



