Faraday to Peter Henry Berthon   16 November 1861

Royal Institution 16 Novr. 1861.

Sir

According to Mr Prossers statement (4 Nov 1861) the actual expence at the South Foreland light house for 12 hours is¹

This sum therefore has to be compared with the cost of a four wicked central lamp in the matter of oil, cottons, &c, for a year - That cost I do not know - I have considered the two lamps as standing against each other in their first cost & in repairs[.]

I have no means of chequing the above estimate:- experience (i.e. practice) only can do that. At the same time I may remark on Mr Prossers statement that 3 cwt of manganese will not produce 216 cubic feet of oxygen at the rate of 63 c:feet per cwt for nearly half a cwt more would be required[.]

I ought to say also - that Mr. Matthey² of Hatton Garden, who has to prepare oxygen in the large way for the fusion of Platinum, makes the price per cubic foot nearly four times the Prosser price:- his Manganese & his labour are both four times the price; but I shall know his result more accurately in a week or two[.]

Nothing has been yet put down for the buildings for the extra man or men. The Gas holders are in the open air. I think it possible that they may require enclosing in winter, both to shelter them from storms & snow, and to keep the water & the pipes from freezing. I cannot consider the apparatus as now left & the efficiency & expence of the whole system as proved until a winter has passed over[.]

It is very difficult to form an estimate of the power of the light[.] Two lights seen at once can be compared together, and the better one, I think, readily determined. But if one is much better than the other, or if one only is seen at a time, then the determination is very difficult, because of the extreme variation of the sensibility of the eye, & the uncertainty of its determination.

Judging from my impressions at different times, both in the house & out of the house, on land & at sea, I think that the lime light, seen from the full front, is equal to two central lamps and perhaps, approximates when the jets are in their best state, towards the power of three. But this is uncertain as the lights have never yet been compared together in the same room³.

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

P.H. Berthon Esqr | &c &c &c