Faraday to Jacob Herbert   29 June 1853

Royal Institution | 29 June 1853

My dear Sir

I was in the Isle of Wight on the 21, 22, & 23 instant examining the condition of the lighthouses at St Catherines and the Needles in respect of condensation & ventilation¹; and now beg to offer you the result of my enquiries. In reference to the St. Catherines light, I will remind you of a report which I made dated 10, Feby 1841², in which the damp state of the tower and of the air in it was adverted to; the description there given still applies; & I have no doubt in certain states of the weather with great force. The SW side of the tower is stained with dampness within from the top to the bottom as if the prevalent wet weather of that quarter could penetrate the stone work. The keeper says that there is frequently a haze in the tower when there is none outside. He reports also that he has occasionally taken up three or even four buckets full of water which have condensed within the tower & run down the walls. These circumstances show that in certain conditions of weather & temperature the tower & its walls must be quite damp, & the air within it saturated with moisture. Yet this is the only air which has access to the lanthorn to ventilate it and when the glass or roof of the lanthorn are by any external change of temperature rendered colder than the tower at such times as these condensation must ensue there. I found at my visit on the morning of the 22nd with moderately fine weather that the dryness within the the [sic] tower was indicated by a hygrometer temperature difference of only 7˚, whereas outside the difference was 11˚.5. The keeper reports that occasionally when symptoms of condensation have come on in the lanthorn he has hung up a piece of dry flannel or a sheet of blotting paper in the current of air entering from the tower & they have become quite damp in a short time. The condensation is still very much better than it was before the lamp ventilation flue was erected[.] It begins first on the cold metal of the roof & from that runs down: sometimes it occurs also on the glass.

All these results prove what I have urged on former occasions that a lanthorn should have its own ventilators or air passages[.] If this lanthorn were supplied with eight good large ventilators one in each face or a larger number of smaller one[s] equally disposed on different sides, the tower might be continually shut off by the doors (which are up) and I am prepared to expect that the evil now existing would be in a great degree or entirely removed. The ventilators would have to be fixed in the iron sides of the lanthorn; there would indeed be room in the glass without interfering with the light, for the glass extends 8 1/2 inches below the lowest reflecting zone, but their position would be better lower down & in the iron than in the glass, & the appearance also better[.]

The cowl is so formed & placed as to produce I suspect an effect here like that discovered & remedied at the Needles light house[.] It is part of a ball 2 feet in diameter revolving on a truncated cone about 2 feet high which stands on the summit of a conical & ribbed roof. The wind which drives against this roof must on the windward side be driven up & enter into the cowl between it & the cone and in such cases will retard the exit of lanthorn air there and may even make a return into the lanthorn in which case condensation is almost sure to occur. The keeper reports that he has occasionally traced the air of the lanthorn backwards into the upper part of the tower which I can only account for by some such return effect as that I am describing. I verified the facts at the Needle light house (See report 30 June 1849³) and the evil was perfectly remedied by the elevation of the cowl & the application of a deflector plate or ledge round the opening. I could not get to the outside of the cowl of the St. Catherines light to make a similar examination there. I should be very glad to see the cowl a couple of feet higher from the roof[.]

The very form of the cowl & cone on which it stands tends to make the wind drive in at the angle or opening a at which they meet, and it is not easy to apply a deflector plate here: for if it be applied on the cone the globular form of the cowl would tend to direct air in between; & if applied on the cowl it would only make matters worse. I should be very glad if a wind guard could be tried once practically instead of a cowl. I believe it would much surpass the cowl in action and help powerfully in windy weather to make a draught through the lanthorn. Such is the case every where with the many smaller wind guards put up in connection with the stove pipes.

The lanthorn stove pipe here is 50 feet long of which 25 feet are horizontal. Yet having no fire in it, there was an excellent draught in every part & at the ash pit of the stove because of the wind guard with which it is furnished. I was able to discover the very part of the pipe within which the keeper had stuffed a duster & then forgotten it; by the action of this cold draught[.]

The last 20 feet of this pipe is vertical and because of the warming power of the previous horizontal part on the air of the lanthorn, it is comparitively cool when the fire is alight, & the water produced by the combustion of the coals condenses in it. The joints are made by putting the upper pieces over the lower & this causes the condensed water to be conducted outwards at every point. The whole of the vertical part should be inverted that the water may run inwards; and the bottom stopper there should be a vessel to catch this water, that it may be removed daily or as it is produced.

The keeper here has ventured as I understand without authority to alter the ventilation pipe associated with the burner; he has turned one of the funnels upside down and then closed the aperture. By this he has increased the length of the lamp chimney to 8 feet 6 inches whereas the Standard or Fresnel⁴ height (which I have never interfered with) is only 5 feet. In trying to find any sound or practical reason for this departure from the use of every other like light house I could not find that the keeper had any distinct reason but he considered the result good: but the following occurred to myself. Whenever the condensing condition comes on the keeper shuts off the air of the tower as much as he possibly can: he tells me the light then turns red & foxy and smokes, and I believe he gets the lanthorn into such a state that there is an important increase of carbonic acid & diminution of oxygen. Under such circumstances, to lengthen the chimney would be to increase the draught & to brighten the flame; and I suspect this is what has led to his change. Of course at other times he is using this increased draught & I conclude the consequence is that he is burning more oil than other three wicked lamps. The fourth or middle wick is taken away; but there could be no occasion to remove it from a lamp having a chimney 8 feet 6 inches high. I have no objection in the present case to this increase in regard to ventilation but I would much rather that the funnel or expansion were removed & the whole length of pipe made straight[.]

At St. Catherines all was right in the lanthorn & the ventilation perfect. At the last painting even the slates of the roofs were covered with white lead[.] I think this is not expedient. Above five sixths of the paint has been washed off by the rain and carried into the tanks to the injury of the water and the roofs look very bad⁵[.]

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

Jacob Herbert Esqr | &c &c &c