VOL. LXIII.] PHIXOSOPHICAL TRANSACTIONS. 45'/ 



indeed the silk, when of considerable breadth, in some measure effected; but he 

 thought that this obstruction to the immediate return of the fire might be ren- 

 dered more complete by increasing the thickness of the silk, or by applying to 

 it some nonconducting substance, that might confine the excited fluid more 

 perfectly to the surface of the revolving cylinder. Bees-wax being a nonconduct- 

 ing substance easily procured, he rubbed the silken flap with it, and found that 

 the return of the fire to the cushion at the anterior part of the machine was by 

 that means much diminished, and consequently the excitation of the glass was 

 apparently increased. The addition however of more silk was still more effec- 

 tual, in confining the fire to the glass; and when it was employed 10 or 12 times 

 doubled, it seemed to deny any passage from the glass to the cushion. 



As Dr. N. thus discovered the method of remedying the common defect in the 

 construction of the anterior part of the cushion, he next attended to that part 

 which corresponds with the descending side of the cylinder. Being convinced 

 that this part of the rubber was alone concerned in the excitation, he imagined 

 that the reverse of what was necessary anteriorly should be adopted in the struc- 

 ture of the posterior part ; that instead of placing nonconducting substances be- 

 tween the glass and cushion, we should here make the afflux of the electric 

 matter as great as possible, by the application of the most perfectly conducting 

 bodies. Confining therefore the amalgam to that place where the glass first 

 comes in contact with the rubber, he placed some tinfoil close to the amalgam, 

 and bending it back, secured it to the metallic plate below the cushion. By this 

 means the electric matter found an easy access to the place of excitation ; and 

 the eflfect of the machine was thereby greatly increased. A piece of leather, 

 covered with amalgam, and fixed to the posterior part of the rubber, in such a 

 manner as to allow about an inch of it to pass under the cylinder, answered 

 every purpose of the tinfoil ; and, as it was not liable to be corroded by the mer- 

 cury, like tinfoil, it was on that account much preferable. 



From the above experiments it was apparent that the excitation was altogether 

 performed by the posterior portion of the cushion ; and that the anterior part, 

 when made of conducting substances, re-absorbs the greater quantity of the ex- 

 cited matter. In the structure therefore of electrical machines, we should al- 

 ways have a free electric communication behind, to facilitate the excitation; and 

 the most perfectly nonconducting substances before, to prevent the re-absorption. 

 To answer these intentions, it will perhaps be advisable to make the cushion of 

 silk, stuflTed with hair, and to lay some metallic conductor round the posterior 

 part, that a free access may be allowed to the electric matter coming to the place 

 of excitation from the inferior part of the machine. Cushions, made in this 

 manner, and then covered with silk 10 or 12 times doubled, are much more 

 powerfully excitant than any others that he had yet tried. Various other methods 



VOL. XIII. 3 N 



