Faraday to Trinity House   10 February 1841

To the Master¹, Deputy Master² & Elder Brethren of the Trinity House

My Lords & Gentlemen

Having been down on the 2nd, 3rd & 4th instant to the St. Catherines light house³ with Captain Madan & Captn Weller⁴ I at the request of Captn Madan beg leave to make a report on the principal results of that visit[.]

The principal object of the visit was to ascertain the general effect of a peculiar chimney applied to the internal light at my suggestion; to observe its power in preventing what is technically called the sweating of the lanthorn - and to examine generally & if possible prevent the great tendency to the deposition of dew or ice on the windows which has hitherto existed[.]

The severity of the weather combined with other circumstances had brought the lanthorn into such a state that I found the inside surface of the glass & the interior of the copper roof coated with ice differing in thickness from that of a sheet of paper to half a crown or more; and the great extent of the evil thus caused was sufficiently evident. For in the lanthorn there was on the first night I was there a great flood of light the glass being like whitened walls but on the outside there was a corresponding diminution from the interference of the coat of hazy ice.

I will first refer to the water from the flame & its consequences. The composition of Spermaceti oil is nearly 78 Carbon, 12 Hydrogen, 10 Oxygen, per unit and when such oil is burnt 100 parts by weight inevitably produce 108 parts of water or above its own weight the increase being from an element supplied by the air. So if in a long winters night a central lamp were to burn 2 gallons of oil & the results of the combustion were to be delivered into the general space of the lanthorn it would be equivalent to throwing in 2 gallons of water in the state of vapour and if the air outside were cold or damp it is hardly possible but that less or more of this must be condensed on the inside of the glass[.]

The object I had in applying a lanthorn chimney to the lamp was first to remove this water out of the lanthorn as soon as formed and next to assist in ventilating the place but in effecting this it was equally important not to interfere with the burning of the lamp,- or to cause by accident any down ward draught on to it; or to obstruct the issuing rays.

The chimney which has been applied consists of three lengths having generally the form & arrangement in the figure and it acts most perfectly. It is not wider than the lamp chimney except in the funnel apertures and yet it carries off all the results of the combustion of the lamp & there is besides draughts of air from the lanthorn up a b and c which with what goes up through the lamp burner itself is more than enough to ventilate the place[.]

In the lowest, and part of the next, length of the lanthorn chimney the soot always set free from the flame in greater or smaller quantity even when it is burning in its best condition was found to be effectually caught and thus the mirror refractors, the glazing of the lanthorn & the inner surface of the roof will be kept much cleaner & brighter than they could be if the chimney were away[.]

The particular construction of the chimney with the arrangement of the openings was especially intended in no way to affect the burning of the lamp & on this point I was very anxious. Here also our success was perfect[.] By a valve we could shut the lowest length of the lanthorn chimney - but on trial whether it were open or closed it did not appear to affect the flame[.] Of course when closed the results of the combustion came into the lanthorn; but whether open or closed the combustion at the wicks and consequently the light to the Mariner went on undisturbed[.]

If from any accident the ball at the top of the lighthouse became fixed & its apertures were to windward so that the wind were to blow into the lighthouse there although it would throw the chimney out of use to a greater or smaller degree it would not interfere with the burning of the lamp or even make the lanthorn any worse than if the chimney were away.

As regards therefore the water of the flame I think the remedy is perfect & accompanied with many contingent advantages.

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But the circumstances of the lighthouse appeared to indicate that the tower sent a great deal of moisture into the lanthorn and I therefore set about examining this point[.]

There is no door between the tower & the lanthorn[.] I therefore hung up curtains in what might be made a door way & found a very powerful current of air passing from the tower into the lanthorn[.]

I next examined this air for moisture for which purpose I filled a bottle with ice & water & added a little salt to bring the temperature to about 30˚ and putting this into the current of air just where it left the tower the bottle was soon rendered dim from the dew deposited upon it. This was when the outside thermometer was 25˚ or 26˚.

Such circumstances must of course be very injurious to the effect of a lighthouse, for the lanthorn becomes a condenser of the moisture which is continually passing into it from the tower: for if the glass bottle cooled to 30˚ would cause deposition of moisture from the tower air much more would the glass windows cooled to 25˚ or 26˚ by the external temperature do so. This evil is one the extent of which increases in proportion to the increase of ventilation caused by letting air into the lanthorn from the tower[.]

Perhaps a momentary difficulty may occur in comprehending this point for ventilation is considered generally as a remedy for the evil under examination. But everything depends upon the circumstance whether the air admitted by the ventilation is as regards the lanthorn a drying or a moistening air. Dry air passed through a lanthorn will help to clear of[f] moisture but moist air will, if the lanthorn or the glass of the lanthorn be colder than the air tend to deposit moisture. It is to be remembered that the temperature of the air in the lanthorn is by no means necessarily the temperature of the glass for when there is wind the latter is governed more by the external than the internal temperature & whilst I was in the lanthorn the temperature within being in different parts 46˚, 50˚, 60˚ &c from the action of the stove[.] Still nearly the whole of the glass was much below 32˚. Now the air of the tower going into a place warmer than the tower would tend to dry up moisture but going into the lanthorn & against the glass which is so much colder would produce exactly the reverse effect & deposit moisture[.]

I endeavoured by canvas hangings to shut off the tower from the lanthorn and the good effect produced by this imperfect expedient made an enormous difference in the state of the lanthorn on the ensuing night so that we were able to close off by far the greatest portion of air from the glass & the lanthorn presented a very different appearance to that of the previous night[.]

Hence I came to the conclusion that it was of great consequence to have two doors one from the tower into the room under the lanthorn & the other between that room & the lanthorn: this I believe was so much in accordance with Captn Madans judgment that he ordered the doors. As soon as they are up & the present moisture cleared out of the lanthorn (if not already gone from the changes we made whilst present) I have no doubt it will remain clear & bright whatever the state of wind & weather[.]

In a lighthouse the glass of the lanthorn will generally be the coolest part: and will consequently be the first place where condensation of moisture will appear[.] Hence upon principle the great points are to keep out all extraneous moisture, as by shutting off the tower if it have damp or pervious walls & by making the walls & surfaces of the lanthorn & room beneath impervious to external moisture either by constructing them of metal or painting them well; and to carry out the moisture which the flame of necessity generates. After that, there remains only the portion which the men produce by respiration &c & this will be far more than compensated for by the ventilation effected through the medium of the lamp & its chimney[.]

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The complete action of the chimney notwithstanding the disjunctions at the funnel shaped parts is very important not only in the present & similar cases but as giving the power of applying the same system to moving lights and I see no difficulty (though of course there would be more complexity) in carrying off the water from every one of many separate argand burners arranged in a revolving reflecting light or indeed from any arrangement of lamps which it might be needful to establish in a light house⁵[.]

I have the honor to be | My Lords & Gentlemen | Your Obedient Humble Servant | M. Faraday

Royal Institution | Feby. 10th 1841