Faraday to Jacob Herbert   13 March 1848

Royal Institution | 13 March 1848

My dear Sir

I have carefully considered the paper you sent me respecting the Coquet light house¹ and herewith send you my observations upon it. I return the paper the paragraphs of which I have numbered in red ink; my observations are numbered in black to correspond with them. I should be glad of a copy of the paper to place with the copy which I keep of my own remarks. The paper has recalled to my mind many of the points which arose when I was last at the South Foreland upper light² and I cannot help but think now as then that the best & cheapest remedy in such cases is to cut off the lanthorn from aereal connection with the building & places below. I do not know to what extent the peculiar circumstances of lighthouses may raise the expence of any such process but I should think that for a building under ordinary circumstances on shore & accessible in the usual way a few pounds would have been enough for the purpose. I beg again to recall your mind to the report on the South Foreland lighthouse (1 Feby. 1847)³ which I drew up with great care because I thought the circumstances were probably the same at many other places[.]

I am My dear Sir | Very Truly Yours | M. Faraday

Jacob Herbert Esq | &c &c &c

(1.) Water is produced in a lanthorn by only one cause namely the combustion of the oil but the air which enters the lanthorn carries in other water in the state of vapour. The latter is more or less abundant under different circumstances but is in my opinion greatly under command.

<⁴> Very doubtful.

<⁵> Because every particle of the air which enters the lower lanthorn must come in contact with the cold glass & remain in contact with it until cooled to the temperature of the glass before it enters the upper lanthorn or else water would deposit upon the glass in the upper lanthorn. Now under ordinary circumstances not a tenth part of the air passing through the lower lanthorn would pass in contact with the glass or undergo this cooling[.]

(4. 5.) These tubes would be much better than the suppositious lower lanthorn of 2 & 3 but are not so good as simply taking the air from the outside at once as is done already in several arrangements;- and cutting off the lanthorn entirely from the air of the tower as proposed in my report on the South Foreland lighthouse. For it is manifest that air taken from any place & led through tubes to be cooled by the air on the outside & so made to deposit moisture could never by such a process become colder than the air on the outside; and therefore not drier[.] If the air were drier before hand then of course it would remain drier afterwards, but it would not then deposit moisture in the tubes or in the glass and the case does not enter into those under consideration[.]

(6. 7.) I do know the circumstances of the Coquet lantern but the description indicates almost certainly that the air which enters the lanthorn is in a great measure that which has previously gone through warmer parts of the tower & building where it has dissolved water with air on coming in contact with the cold glass of the lanthorn deposits the water it had before taken up. I do not think that air taken at once from the outside into the lanthorn would do this & I earnestly beg again to refer to the principles contained in my report upon the South Foreland lighthouse[.]

(8.) Good in principle. But if the lanthorn be cut off from any air communication with the tower as I have recommended in my report: then I think that the air may be as well brought into the tower by ventilating or air holes at the sides of the lanthorn as by a tube at the top and I think this will not be more necessary & convenient because the persons in the lanthorn require some supply of air as well as the light.

(9. 10. 11.) Generally correct. But there is a source of cold which is not adverted to in the paper namely radiation which is very influential in a clear starry night in winter, and, as we well know, especially influential at the top of things as trees & buildings raised into the air. How far therefore the warmth would be sufficient at all time without a stove, I cannot say. I should doubt it.

M. Faraday