CHAP. V.] OPENING MANHOOD - 1847 TO 1850. 129 



the three laws of Liberty, Equality, Fraternity, thus ex- 

 pressed : 



1. That which can be done is that which has been 

 done; that is, that the possibility (with respect to the 

 agent) of an action (as simple) depends on the agent having 

 had the sensation of having done it. 1 



2. That which ought to be done is that which (under 

 the given conditions) produces, implies, or tends to the 

 greatest amount of good (an excess or defect in the variables 

 will lessen the good and make evil). 



3. Moral actions can be judged of only by the principle 

 of exchange ; that is (1), our own actions must be judged 

 by the laws we have made for others; (2), others must 

 be judged by putting ourselves in their place. 



March. 



At Practical Mechanics I have been turning Devils of 

 sorts. For private studies I have been reading Young's 

 Lectures, Willis's Principles of Mechanism, Moseley's Engineer- 

 ing and Mechanics, Dixon on Heat, and Moigno's Repertoire 

 d'Optique. This last is a very complete analysis of all that 

 has been done in the optical way from Fresnel to the end 

 of 1849, and there is another volume a-coming which will 

 complete the work. There is in it besides common optics 

 all about the other things which accompany light, as heat, 

 chemical action, photographic raj^s, action on vegetables, etc. 



My notions are rather few, as I do not entertain them 

 just now. I have a notion for the torsion of wires and 

 rods, not to be made till the vacation; of experiments on the 

 action of compression on glass, jelly, etc., numerically done 

 up ; of papers for the Physico-Mathematical Society (which 

 is to revive in earnest next session !) ; on the relations of 

 optical and mechanical constants, their desirableness, etc., and 

 suspension-bridges, and catenaries, and elastic curves. Alex. 

 Campbell, Agnew, and I are appointed to read up the subject 



1 Maxwell often insisted on this in conversation, with especial 

 reference to our command of the muscles depending on experience of 

 the muscular sense. 



K 



