458 PHILOSOPHICAL TRANSACTIONS. [aNNO 1773. 



however may be pursued in the construction of the rubber; but it should be an 

 invariable rule, to place nonconducting bodies before, and conducting substances 

 behind, the cylinder. From the preceding principles, it follows, that the sup- 

 port to the rubber should likewise have its conducting and nonconducting side. 

 For this purpose, it may be necessary to employ baked wood, and to cover the 

 posterior half with tinfoil. The place of excitation will be thus sufficiently sup- 

 plied with electric matter, and the cylinder will not be robbed of a part of the 

 excited fire, before that fire has made a revolution with the glass. 



By attending to the place where the excitation is effected, it must appear evi- 

 dent, that the amalgam is only to be laid on the posterior part of the cushion ; 

 its presence indeed would be useless, if not injurious, in any other situation. It 

 will however be found somewhat difficult to confine the pure amalgam to the 

 posterior part of the rubber ; but if it is mixed with a little hair powder and 

 pomatum, it pretty well keeps its place. The strewing the amalgam thus pre- 

 pared on the glass, as it revolves, is perhaps the best method of applying it; as, 

 by that means, it is in a great measure prevented from passing on to the non- 

 conducting substances that are placed before. Should any of the amalgam be 

 carried forward by the revolution of the glass, it should be carefully removed. 

 The necessity of keeping that part free from conducting bodies cannot be too 

 much insisted on ; and when fresh amalgam is applied as before mentioned, to 

 the proper part of the rubber, the flap should be held down during half a dozen 

 turns of the machine, lest it might collect some of the amalgam before it is pro- 

 perly fixed. It is a probable conjecture that, when the flap of silk is covered 

 with amalgam, part of the amalgam, which is not immediately subservient to the 

 lexcitation, acts as a conductor in restoring the fire again to the cushion; and 

 that thuSj by an improper disposition of it, we suppress, instead of increasing, 

 the quantity of the excited matter. 



In short, when an electrician attends to the preceding principles in the con- 

 struction of his rubber, and to the proper disposition of the amalgam, he has 

 nothing to fear from the humidity of the atmosphere, as his machine will work 

 equally well in all kinds of weather. The rest of the electrical apparatus may be 

 made according to the directions that have been given by the different electrical 

 writers. Each has had his favourite machine; and perhaps no one has been yet 

 contrived that has not had its peculiar advantages. 



XXXVI. Properties of the Conic Sections; deduced by a Compendious Method. 

 Being a Work of tJie late Wm. Jones, Esq., F.R.S., which he formerly com- 

 municated to Mr. J. Robertson, Libr, R. S., and by him addressed to the Rev. 

 N. Mashelyne, F.R.S., &c. p. 340. 



It is well known that the curves formed by the sections of a cone, and there- 



