Faraday to Jacob Herbert   14 September 1846

Royal Institution | 14, Septr. 1846.

My Dear Sir

I paid a visit on the 4th instant in the day time to the South Foreland high light¹ to examine generally into the probable causes of the dampness of the windows of the lanthorn which occasionally occurs at night:- This examination was only preparatory, inasmuch as a visit on some night in cold weather will be needed when the effect is actually taking place². Still certain results seem already to indicate the cause of the effect & I therefore trouble you with these remarks[.]

In the first place I found the apparatus for the ventilation of the lamp in good condition & I think not likely to allow of any return of the water formed by the flame into the lanthorn; provided a proper moderate entrance of air into the lanthorn to ventilate it be allowed. This I shall ascertain on a future night examination[.]

In the next place the air of the lanthorn, the room beneath & the whole tower felt to me as if moist & in that respect probably sufficient to account for the effect; I therefore proceeded to examine both the internal & external air by an Instrument (Daniells hygrometer) which tells us the dampness of air by shewing at what degree of temperature it begins to deposit moisture. I found the temperature of the open air without the tower in the shade was 66˚ and this air required to be lowered to 57˚ before it deposited moisture[.]

In respect of the air within the tower & its parts

The air in the room below the lanthorn deposited water at 60˚

The air in the out chamber <-> D˚ <-> 62˚

The air in the lanthorn itself (receiving from

at 62˚ and 63˚

the tower & the passages of the central stove[)]

Hence a great difference is manifest in the dryness of the air within & without the building & hence a great tendency of the air within the building to deposit moisture in a case of coldness - for imagine the glass of the windows to be cooled down to 60˚; the natural air on the outside of the tower would have no tendency to condense water on it at that temperature but the air contained within the lanthorn & tower would deposit moisture abundantly[.]

In further support of this view of the cause of the dampness on the windows I may state that the keeper Finnis³ says that the windows occasionally become damp on the inside in the day time before the lamp is lighted shewing that even in the day the windows may become so cool as to condense moisture from the air within. Again if 2 or 3 persons go into the lanthorn in the day time & remain there only a very short time the windows will often become dim; shewing that the air is so moist a very little addition from the breathing of 2 or 3 persons will bring it up to the saturated condition & then it deposits on the windows.

This excess of moisture in the air within the building above that which is present without is I think communicated to the air from the building itself. This tower was new about 3 years ago & as the keepers do not inhabit it has become so dry as an inhabited building would have done in the same time and indeed being uninhabited it cannot I think ever become so dry as if it were occupied by the keepers. Further it is more slow in drying & less likely to acquire that degree of dryness which it might otherwise attain to because the keepers avoid as much as they can the use of the stove finding that the windows damp more when it is in use and this is likely to be the case as the lower ends of the air passages are in the stone or brick material of the building & the air passing through gathers moisture from them[.]

These views which are in some respects premature I propose verifying or correcting as soon as the keeper informs me the season for the effect in question has come on. If they are correct I would suggest as remedial measures the exclusion of the air of the tower from the lanthorn by doors &c & the use of a stove in the lanthorn itself to sustain the temperature of the glass of the windows⁴.

I am My dear Sir | Your Very Faithful Servant | M. Faraday

J. Herbert Esq | Secretary | &c &c &c