ON THE STUDY OF PHYSICS 299 



central cause on which they depend. But, having guessed 

 the cause, we are not yet contented. We set out from the 

 centre and travel in the other direction. If the guess be 

 true, certain consequences must follow from it, and we 

 appeal to the law and testimony of experiment whether 

 the thing is so. Thus is the circuit of thought completed 

 from without inward, from multiplicity to unity, and 

 from within outward, from unity to multiplicity. In thus 

 traversing both ways the line between cause and effect, 

 all our reasoning powers are called into play. The mental 

 effort involved in these processes may be compared to 

 those exercises of the body which invoke the co-opera- 

 tion of every muscle, and thus confer upon the whole 

 frame the benefits of healthy action. 



The first experiment a child makes is a physical exper- 

 iment: the suction-pump is but an imitation of the first 

 act of every new-born infant. Nor do I think it calcu- 

 lated to lessen that infant's reverence, or to make him a 

 worse citizen, when his riper experience shows him that 

 the atmosphere was his helper in extracting the first 

 draught from his mother's breast. The child grows, but 

 is still an experimenter: he grasps at the moon, and his 

 failure teaches him to respect distance. At length his lit- 

 tle fingers acquire sufficient mechanical tact to lay hold of 

 a spoon. He thrusts the instrument into his mouth, hurts 

 his gums, and thus learns the impenetrability of matter. 

 He lets the spoon fall, and jumps with delight to hear it 

 rattle against the table. The experiment made by accident 

 is repeated with intention, and thus the young student re- 

 ceives his first lessons upon sound and gravitation. There 

 are pains and penalties, however, in the path of the in- 

 quirer: he is sure to go wrong, and Nature is just as sure 



