126 I^aclid' s Eletnetits : Electrical Excitation 



high tension is by no means essential to the production of 

 colour, though of course a proportionally greater thickness 

 of the medium must be employed to produce a similar effect 

 when the elasticity is small. 

 Glasgow, Dec. 29, 1838. 



XXII. Rcmar/cs on a imper in the Philosophical Magazine 

 for December 1838, o« a certain demonstration of Euclid. 



By A CoKRESPONDENT. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 

 TN this month's number of the Philosophical Magazine (Dec. 

 -»- 1838. vol. xiii. p. i^'t) one of your correspondents has given 

 a method of dispensing with the 12th axiom of the 1st book 

 of Euclid's Elements, apparently not being aware that Pro- 

 fessor Peacock has given the same in his treatise on algebra. 

 It would, indeed, be immediately suggested by a perusal of 

 the demonstration of tiie 12th axiom, and the remarks upon 

 it by Ptolemy, quoted by Proclus in his commentary on 

 Euclid's Elements. 



It appears to me to be open to the objection which Proclus 

 makes to the emendations of iEneas Hierapolites, and others on 

 the Elements, "for the geometrician appears to have chosen 

 such hypotheses as either abound in affirmation or are more 

 simple." Setting aside other more weighty objections, Pro- 

 fessor Peacock's proposed amendment, although it removes 

 one difficulty, is certainly not sufficiently simple to be placed 

 among the definitions of the 1st book. When I say not suf- 

 ficiently simple, I mean that it does not immediately impress 

 upon one's mind the common idea of the subject defined, an 

 object to which Euclid has always been careful to adhei'e. 



Your obedient Servant, 

 Jesus Collie, Cambridge, Dec. 1st, 1238. J. O. H. 



XXIII. Notice of the Electrical Excitation of a Leather Strap 

 connecting the Drums of a Worsted Mill ; in a letter to Dr. 

 Faraday y}'07?i the Rev. T. Drury*. 

 My Dear Sir, 

 pERMIT me to describe an extraordinary electrifying ma- 

 chine which I yesterday witnessed, and which I think 

 will be new even to you. 



* Communicated by Dr. Faraday. 



