Faraday to Jacob Herbert   14 October 1848

Royal Institution | 14 Octr. 1848

Sir

In reference to the subject of your letter of the 7th instant¹ I have to state that I have been at Messrs. Wilkins Manufactory² & examined a lanthorn of the larger size referred to. The other or smaller lanthorn was not there but I saw parts of one and understood that it was in every respect like the larger as to shape construction & horizontal dimensions but was shorter. In the large lanthorn the glass was 43 inches in height in the smaller only 28 inches high. I understand also that the lights are placed upon one level i.e that whether 8 lamps & reflectors were put into the larger or the smaller lanthorn they would all be on the same plane & not one above the other. The entrances and exit of air also are in both cases the same in form size & disposition. The question then being whether the smaller lanthorn is sufficiently capacious for the eight lamps & as good in that respect as the larger one I must in the first place say that it is a very practical question & can only be finally & certainly answered by trial but after making this reservation I am free to say that I think it will. The lamps do not depend for their supply of air on the capacity of the chamber or lanthorn but on the amount of ingress & egress & if they be alike in both cases there will in both cases be an equal supply of air to the lamps. The capacity may be influential in regulating in some degree the gusts & blasts of wind but any one of eight lamps in a given lanthorn is just as liable to that disturbing cause in company with the rest as if it were burning alone in the same lanthorn all other things being equal. If therefore four lamps on one level have been found to burn well in a certain sized lanthorn like that I examined in part at Messrs Wilkins I expect that eight lamps placed on the same level in the same lanthorn will burn as steadily because the entrances & exits for air are equally guarded in both cases: and as well because by the number of apertures a double supply of air is given in the latter case[.]

I am Sir | Your Very Obedient Servant | M. Faraday

J. Herbert Esq | &c &c &c