REVIEWS — ANALYTICAL STATICS. 69 



finite minds take cognizance — that our process is to collect laws 

 from observed facts, and then to trace out the remote consequences 

 of those laws : and that consequently our results, whether account- 

 ing for or predicting phenomena apparent to us, may be depended 

 upon, however metaphysical speculations might interfere with the 

 objective correctness of our assertions. This answer we clearly 

 cannot make if our belief of the inertia of matter in any way de- 

 pends upon our persuasion that " space is infinite in extent and 

 every where identical." Again, if the principle in question is to be 

 established by an appeal to experience, it must be made in a much 

 more guarded manner. When we speak of a body passing from a 

 state of rest to a state ot motion, both the rest and the motion 

 must be relative : and it should at least be pointed out that we are 

 obliged to draw an inference concerning a particle absolutely at rest, 

 from the examination of a body relatively at rest. And when we 

 come to consider the case of a body passing from a state of relative 

 rest to one of relative motion, it is necessary to guard our language 

 hy another restriction, which may tend to increase the embarrass- 

 ment of the learner. The cause to which the change of state is to 

 be referred may be one applied either to the body observed or to 

 the system relatively to which its state of rest or motion is estima- 

 ted. A carriage is suddenly stopped and a person riding in it is, 

 to use the popular language, thrown suddenly forward. He pas- 

 ses from a state of rest to a state of motion with regard to the car- 

 riage, exactly because no force is applied to him, and the case is an 

 illustration, not of the statical but of the dynamical aspect of the 

 principle of inertia, viz., that the body once in motion will continue 

 to move unless some external cause be applied to stop the motion. 

 So that if the statical principle of inertia is to be established by an 

 appeal to experiment it must be in language somewhat more ex- 

 tended than that used by our Author, unless, indeed, his book is in- 

 tended merely as a peg to hang lectures on. Before quitting this 

 point we would state that our own impression is in favor of treating 

 these fundamental principles, in the case of beginners, not exactly 

 as axioms, but as facts which the learner must for the time take 

 on trust. It is not until the mind has become familiar with the 

 ideas of force, and of rest and motion, absolute and relative, that, 

 as a general rule, it can take in the train of thought upon which such 

 principles depend. It seems to us to be not only the easier but the 

 safer course (and we would suggest this as a practical consideration 

 to those of our readers who are engaged in teaching) to assume, and 



