Faraday to Jacob Herbert   19 December 1843

Royal Institution | 19 Decr. 1843

Sir

On the 15th & 16th instant I went to the Lighthouses at the South Foreland to examine the state action & effect of the ventilating systems applied to the lamps of these lighthouses respectively[.]

In the lower house the light is that of many Argands each in a separate reflector. To these the divided system of papers has been applied & the workmanship executed in a very efficient & satisfactory manner by Mr Wilkins. It had been in operation 5 days & its effect, as far as I could observe it, excellent. When I looked at the light from the outside I could scarcely perceive that there was glass in the frames it was so clear from dew or anything like it and when I went into the house the report of the keeper was equally satisfactory. One circumstance which the man described will illustrate the value of the ventilating system. The cowl at the top of this lanthorn is an old one & does not turn with a very light wind: the consequence was that the cowl occasionally stood with its opening to the wind & the latter even when light was sufficient in power to prevent the air of the lanthorn escaping which consequently became impure & to such a degree that the top lamps would hardly burn whilst on the keeper the atmosphere produced oppressive & heavy sensations. At such times the keeper has gone out & turned the cowl round & this he has often had occasion to do. Since the ventilation system has been applied he has taken an opportunity of turning the opening of the cowl purposely to the wind yet for all that the hot air now came out in a stream the lamps went on being as they ought, the air removed the whole time sweet & good, & the windows bright.

The upper light house has a single central lamp to which the ventilating chimney has been added some months since. It acts and answers well. I found the windows in the lanthorn perfectly dry & clear but those in the tower were all damp, & in respect of the upper ones pools of water which had run down from the glass, were standing upon the sills. From the general dampness of the building consequent upon its newness, it often happens that in the day time not only the windows & walls of the tower run with water; but the windows of the lanthorn, the roof, & the refractors themselves sweat much: in these cases when the lamp is lighted & the ventilating chimney comes into action it always diminishes the quantity of dew & almost always clears it off.

The dew on the glass & apparatus of the lanthorn whenever it appears is altogether occasioned by the wetness of the new tower & the saturated state of the air passing through it¹.

I am Sir | Your Very Obedient Servant | M. Faraday

Jacob Herbert Esq | &c &c &c | Trinity House