Faraday to Jacob Herbert   8 December 1849

Royal Institution | 8 Decr. 1849

My dear Sir

I returned home last night and proceed to give you in brief the result of my visit to the Needles & Portland lights. I found the work at the Needles light drawing to a conclusion and so far advanced that I could decide upon the effect to be produced:- for the ventilators were in place & the cowl had been lengthened and guarded according to my instructions. I am happy to say that the result is perfect and the state of things utterly unlike what it was at the time of my visit in June¹. I fortunately had the best opportunities of examining the effect produced for on Tuesday Night² (when I was there) there was a very hard frost (which in London made the streets very dangerous), and on Wednesday morning about 2 o’clk this changed (with rain) to violent wind at S & by E which is the wind the keeper complains of as the very worst for them. I could hardly keep my feet on the downs on my way from the lighthouse to the Alum bay hotel. Yet both in the time of the frost & of the wind, the glass was perfectly clear & dry and also the inside of the metal roof. There were no signs of condensation or of moisture in the lanthorn. All the lights burnt well and clear. The ventilation was right at every lamp, & a strong current existed out of the lanthorn into & through the cowl the whole time. Even when the lamps were extinguished on the Wednesday morning this current continued well & powerful:- so that there is no chance of any such entrance of air into the lanthorn by the cowl as occurred when I was here before. I believe that the evil which existed is perfectly remedied. The keepers & Mr Wilkins workman testified of the clear state of the glass ever since the cowl had been raised & guarded.

I then proceeded to the Portland lights³ and passed part of Thursday with the night there. As to the lower light I found it in perfect order relative to the ventilation & no trace of dimness or moisture on the glass. The keeper said he had had no sweating since the ventilation pipes had been put up.

At the upper light, the lamps & their ventilation was quite right and though we had during the night & morning a strong gale of wind with rain from the S and SE yet all acted well & the glass remained clear. The keeper reports that nothing is wanting in this respect.

But there is here a stove at the bottom of the staircase tower leading up into the lanthorn, which they have to use in cold weather. But when at such times the wind is between NNE and E the smoke regurgitates into the tower issuing in a dense volume from the ash pit & door of the stove; & it ascends into the lanthorn in such quantities as actually to obscure the lights[.] The keeper spoke of one time when there were two East Indiamen lying to the West (as I understood him) of the light to which he thought of going & stating that as the wind then was they must be prepared for changes in the appearance of the light[.]

After much & careful examination I think I have come to a right conclusion as to the chief & almost sole source of this effect. The stove is surrounded by a jacket which is intended to produce warm air & from this a pipe 12 feet long & 4 inches in diameter passes upwards through the floor of the lanthorn terminating on the upper surface of the floor by a ventilation plate not more than 2 inches in diameter, though the air therefore round the stove is made very hot only a little of it passes by this aperture & the chief effect of the stove in warming the lanthorn is produced by its heating the air in the staircase which then enters the lanthorn by the door. In cold nights the keeper says he has to make the funnel pipe of the stove red hot that he may have sufficient warmth in the lanthorn. It is this funnel pipe which is chiefly in fault. It is 6 inches in diameter and after leaving the stove in a horizontal direction it rises vertically 5 feet 3 inches then passes horizontally through the wall of the tower to the outside being 7 feet in length in that point, (of which 2 feet are inside, 2 feet in the thickness of the wall & about 3 feet outside) and then it rises nearly vertically for about 25 feet more:- being for nearly half that distance against the outside of the small projecting staircase & for the rest of the distance 12 or 13 feet in the open air. The position of this metal pipe is nearly NE of the tower[.]

Now this pipe is exposed to the cooling influence of the wind in all that part outside the tower;- and out of a length of 37 feet. 28 feet are on the outside:- of the remaining 9 feet four are horizontal & there is only 5 feet 3 inches of vertical smoke pipe in the tower which being kept hot can tend to create draught. As to the outside part whenever the wind blows from NNE to E the whole 28 feet are exposed to its chilling influence; and as the stove is only used on cold nights it may be imagined what this influence is. If the wind blows from the opposite side the chimney is protected in part of its course & the effect is not so bad. I believe that when the wind is NNE or to E on a frosty night this pipe is cooled below the freezing point that the smoke within is cooled & being charged with sooty particles & carbonic acid gas is too heavy for the ascensive power of the hot smoke in the 5 feet 3 inches of vertical pipe in the tower & therefore descends & enters the tower through the fire door & ash pit & continues to descend until the tower doors are opened & a counter effect from the wind obtained.

The wind was at S.W when I was there in the evening & therefore I could not expect to find the stove in its particular bad state. Nevertheless I had a fire lighted in it that I might examine the condition of things. It drew very well having the ascensive power of the 5 feet 3 inches of funnel pipe in the tower & also the effect of the sheltered part outside the tower but when I applied my hand to it at the part where it rose above the staircase & left of its shelter I had full proof of the effect of the wind; for though it was not cold at the time the wind was getting up:- & though the pipe was warm or even hot a foot or two below the top of the shelter & the hot smoke continually moving upwards through it as soon as it had risen less than a foot above the shelter it was quite cold.

The keepers tell me the funnel pipe has been lengthened two or three times but this has done no good. Really such additions have done more harm than good for they have only increased the length of the heavy columns of cold smoke within which a wind at NNE or E produces.

The remedy I propose is this:- to dismiss the warm air tube altogether and in its place to take the funnel pipe of the stove up the middle of the stair tower through the stone floor & up the middle of the lanthorn until it rises to the level of the iron cross & then to take it obliquely to & through the roof & raise it about 3 feet above the roof where it shall terminate in a head of which I will immediately speak. In such an arrangement the flue would be kept hot in the building; would warm the air both of the staircase & lanthorn and would not (I believe) consume one third of the coals which are burnt in a cold night under the present arrangement and which amounts at times to a bushel per night[.]

Before I left London I had a certain form of head called a smoke disperser & six feet of a funnel pipe made. These I took with me to experiment with. Removing one of the four old ventilating covers on the roof of the Needles light house I projected this head through about 3 feet. The wind as I have said was a gale but in whatever position I placed the head, turning it in all directions, still its effect (due to the construction) was to create a powerful draught up the funnel pipe from below. I am satisfied this head is an excellent thing. It has no cowl or swinging or moving parts but is a fixture & can be made perfectly firm & stable. When the model comes up from the needles I will send it to the Trinity House & in the meantime recommend it for the head of the stove chimney in the present case.

Should the Board consider my proposition favourably & decide upon making the alteration I propose I have taken all the measurements & positions of the parts and could I think instruct the workmen & Mr. Wilkins perfectly in the arrangement of the pipe &c⁴.

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

Jacob Herbert Esq | &c &c &c