29i Mr. J. Cockle on Light under the action of Magnetism. 



with the instrument ; and that other instruments into whose 

 construction circles entered — such as Astrolabes — would also 

 be furnished with alidades^ and we may perhaps be justified 

 in entertaining some doubt as to the connexion of the words 

 theodolite and alidade. 



It will be noticed that Bourne's "Treasure for Travailers," 

 cited by Mr. De Morgan, was published in 1578, seven years 

 after the first edition of the Pantometria, in which, so far as I 

 am aware, the term athelida does not occur. Might not the 

 term theodelitus, which was prior in point of publication, sug- 

 gest athelida ? 



Adopting, as I do, Professor De Morgan's view, that theo^ 

 delitus is an adjective or a participle, I think that fact favour- 

 able to my own derivation, li^he graduation o^ \he instrument 

 appears to be its principal feature in the eye of the writer of 

 tile Pantometria. " It is," he says, " but a circle divided," . . 

 "or a semicircle parted.." Now if we pass from o/3eXo9, 

 o/SeXi^« to the Doric or iEolic forms oSeXo95 oSeXt^co, we have 

 in the word odelited the very expression of a graduated instru- 

 ment. The prefix The may either be a redundancy, or con- 

 nected with Oedofiat, in which latter case the instrument 

 would be described as a graduated seer, or, what would be 

 equivalent, a seer of graduations or angles. The graduation 

 may, as a criterion, be liable to as much ambiguity as the 

 being furnished with an alidade, but it provides us with a 

 singular approximation in sound and meaning. 



Oti Light under the action of Magnetism. 



At pp. 469-477 of vol. xxviii. of the present series of this 

 Journal, the Astronomer Royal has given equations applying 

 to light under the action of magnetism. Some time since I 

 made a communication, on the subject of Mr. Airy's investi- 

 gations, to a cotemporary Journal*, but the following obser- 

 vations may not be misplaced here. 



Of the species of force to which that alluded to by Mr. Airy 

 in the last paragraph but one of his letter (Phil. Mag. S. 3. vol. 

 xxviii. p. 477) belongs, we have a striking mstancein centrifugal 

 force. Conceive a particle attached to a fixed point by an in- 

 extensible string, and revolving round the point. The tension 

 of the string depends upon the velocity of the particle in the 

 direction of its motion — which is always perpendicidar to the 

 string. This induced me to enter into some investigations on 

 the subject, which I discontinued ; for I found that some re- 

 searches of Mr. O'Brien (in this Journal, vol. xxv. pp. 326- 

 334<) might be adapted to the same end. If (lb. p. 331) we 



* See Mechanics' Magazine, vol. xlvii. pp. 575, 676. 



