Faraday report to Trinity House   12 August 1839

Improvement of Lights.

Professor Faraday’s Report on the Bude Light | exhibited at the Orford Low LightHouse.

To the Master, the Deputy Master¹, and Elder Brethren | of the Trinity Board.

My Lords and Gentlemen,

I have been to Orfordness, where under your direction one of those lamps invented by Mr. Gurney, and spoken well of by me in my last report² to Your Board, has been placed in the lower lighthouse, with the object of ascertaining by actual practical experience whether it is in all points fitted to serve that useful purpose to the maritime interests of this and also other countries for which it was intended.

Heretofore the lower lighthouse, which is furnished with a polyzonal lens and upper mirrors, has had a French lamp placed in it’s focus; but within the last few weeks, this has been replaced by the Bude lamp (which I have in the last report called No. 6 or the Good Lamp). As the great object was to compare the French and Bude lamps, as I had before only had the opportunity of seeing and using one French and one Bude lamp, and as there was an excellent French lamp at the Lighthouse, I immediately commenced experiments of comparison between the Argand, French, and Bude lamps which I shall have the honor first to describe.

Argand Lamp. This was a lamp in very excellent condition, it’s burner being ⅞ of an inch in diameter. It was trimmed and burnt for 12 hours, being attended to by the lighthouse keeper exactly as if in the lanthorn. It’s flame was excellent. It was used for comparison during the day at various times, it’s flame being observed each time to see that it corresponded to a standard length drawn on a card; and at the close of 12 hours it’s consumption of oil was ascertained and found to amount to 0.825 of a pint. The cotton required trimming 5 or 6 times during the 12 hours.

French Lamp. This lamp was also in excellent condition and burnt much more regularly and better than that I had had at the Trinity House, to which it was very superior. The keeper gave it every care and attention, retaining it at a full and steady height of flame for six hours, during which comparative experiments on it’s light were made. The consumption of oil in this time was 6.333 pints being equal to 12.666 pints for 12 hours.

Bude Lamp. This lamp is one that I have not experimented with before and in several points differs from that used at the Pay office in the former experiments. It has 17 jets (each with its surrounding cotton) and these form a circle 3.83 inches in diameter. The diameter of the French lamp outer burner is scarcely 3.5 inches.

Being burnt for 6 hours in as steady a manner as possible the expenditure was of

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The cost therefore of these three lamps in the state in which their lights were compared was, for 12 hours, as follows,

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Light. The comparison of these lamps one with the other as to their light was as follows, the average being the result of from three to five observations in the different cases.

These results are not consistent with each other; for the relation of the Bude to the French lamp is much lower when the Argand lamp is used as a common measure to the two, than it is when they are compared one with the other. This is greatly dependant upon the following circumstances; First the place was exceedingly contracted in extent and very inconvenient for measuring experiments. Next, the time was short, the number of observations few, and therefore the average likely to be in error. Thirdly there is the standing difficulty of the difference in colour of the lights. I am inclined to consider the relation of the French and Bude lights as 1 to 2.5 and to put more confidence in that than in the Argand lamp results; first because the Bude light is nearer to the French light in colour than it is to the Argand; and next because the distances of the lights from the screen are also more nearly alike than when the Argand lamp is used. I have thought it my duty, however, to give the experimental results in the three cases.

Upon the results given the following is the price of light equal to one Argand lamp for 12 hours when obtained by the three lamps; the Argand being used as the Standard,

The following is the price of light equal to 12 Argands for 12 hours obtained by the French and Bude lamps using the French lamp as the Standard.

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The Lighthouse Bude lamp is in several respects not so good a lamp as that I had at the Pay Office; i.e. it is not so economical. This is rendered evident by the amount of light which is obtained for a certain expenditure of oil and oxygen, as illustrated by the comparative experiments already given.

It is rendered evident also by the proportions of oil and oxygen which are burnt in the lamp. I have taken Mr. Hall’s³ report of the lighthouse lamp for the three first weeks and ascertained the ratio of oil and oxygen consumed.

In the first week for each pint of oil 11.3 cubic feet of oxygen was used

Second 11.6

Third 11.84

On the day when I compared the lights at the lighthouse 11.75 c. feet; and on the night of Friday when we observed it from the sea 12.29 c. feet, were used per pint of oil. Now in the former lamp the proportion of oxygen for each pint of oil, was below 10 c. feet; and I gave, in my report the proportion of 1 pint of fuel to 10 c. feet of gas, as being probably the best. As the oxygen forms ⅔ of the current expence of the light, this alteration of course causes an increase in price, at the same time that it indicates an unfavourable arrangement. I am inclined to refer the result to the circumstance that the apertures of the jets are not of the same shape as those in the former lamp; in that case they were carefully made and were, I think, inverted cones.

The lamp has also a broad brass belt about it’s oil tubes which supports a plate serving as a stage to sustain the lamp glass. This prevents the free access of the ascending current of air to the tubes, and they consequently become so hot that the oil occasionally boils in them and burns down the outside; in that respect the arrangement undoes what it was a great object in the former lamp to obtain, namely the refrigeration of the tubes: a point I have specially spoken to in my last report. The evil is increased by the circumstance that all these surrounding plates, as well as the oil tubes, are bright, and so in the worst condition to be cooled by radiation.

The little injurious cones of carbon, formerly spoken of, are produced more readily and frequently on the oxygen jets of this lamp than with the former lamp; a result which is most probably the consequence of the altered shape of the jet apertures and of the heat of the oil tubes.

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I will now enter upon such observations as I could make upon the French and Bude lights at a distance, and from the Sea. In so doing though I may venture to give an opinion on the intensity and visibility of a light or of two lights it is with a full sense of my own inexperience and inability to judge of their fitness for lighthouses; that is a point which I desire to leave to the practical judgment of the Elder Brethren or of Seamen.

There are two lighthouses at Orfordness; and as one of them, namely the upper, is illuminated nightly by Argand lamps and reflectors, in proportion to 24 to the entire circle, it may be made to serve as a kind of practical standard by which the French and Bude lamps, when placed in succession in the lower lighthouse, may be judged. These lighthouses are 4321 feet apart from centre to centre; the middle height of the reflectors in the upper lighthouse is according to measurements made by Mr. Hall and myself, 92 feet above high water mark; the middle height of the polyzonal lens in the lower house is 57 feet; so that their difference in height is 35 feet. The lower lighthouse, besides the polyzonal lens, has five effective tiers of plane mirrors in the upper part of the lanthorn.

The French and Bude lamps were observed from the sea on two successive nights, namely, those of the 1st and 2nd of the present month (August). These were beautiful clear nights and as alike as they well could be, except that the moon which was near her last Quarter, rose later on the second night than the first. The lamps were burnt as nearly as the keeper could judge in the same manner, and to the same degree, as on the day of the comparison of the lights by their shadows.

On Thursday night the French lamp was in the lanthorn. We went out to sea towards, by, and beyond the Ship Wash light. At first the high and low lights were about equal, afterwards to the distance of 12 or 13 miles out, the high light was decidedly the best. We then returned within the Shipwash light, and went towards the S.W.; and, finally, circumnavigating the Whiting Bank returned by day break to our first station off the upper lighthouse. We thus crossed the darker directions of both lights. The high light was occasionally much inferior to the French or low light in intensity but at other times the low light was much inferior to the high light.

On the Friday I replaced the Bude lamp in the lower lighthouse; but in doing so I made certain alterations. The simultaneous observation of the French and Bude lamps during the comparative experiments of Thursday, showed me that the mean horizontal line of greatest light was nearer to the cotton in the Bude lamp than in the French lamp; whereas the two lamps had been so arranged, that, when placed in succession on the table in the lanthorn, their cottons should be at equal heights. Other observations upon the directions of the rays and places of the shadows, convinced me that the Bude lamp was too low in the lanthorn, and that it’s most efficient rays were being thrown into the air above the horizon, at least in two directions at right angles to each other, i.e. to the S.W. and S.E.; if not to the N.E.. I was therefore bold enough, though entirely unacquainted practically with the adjustment of the dioptric apparatus in a lighthouse, to raise the lamp (upon the strength of certain calculations) three tenths of an inch, and there left it for the night.

On examining the plane mirrors I found them also, as I conceived, much out of adjustment for either the French or Bude lamp; and though not prepared for such an event adopted what means I could apply at the moment to their correction. This correction is the same for either of the two lamps; but whatever effect may have been produced by it in the present case, must, in justice to the French lamp, be considered as extra with the Bude lamp; since the French lamp had not the advantage of it, on the night of it’s observation.

On Friday night we went to sea as before, taking the same general direction by the Shipwash light, but going further out. In this direction the lower light was much better than last night; it was always, I think, above the high light in intensity and often much above it. We then returned within the Shipwash light and proceeded S.W. or thereabouts as before. Very soon the lower light shone forth with great brilliancy, and I think must have been as 4 to 1 compared with the upper light; and this continued for several miles until we approached the horizon and nearly had the light down. In this direction it must be remembered we were nearly end on to the lights; the upper being somewhere about 4300 feet nearer to us than the lower. We did not return to the lighthouses but came away home.

From the effect it seems probable to me that the Bude lamp was pretty well adjusted for the Up Channel or S.W. direction; and in fact, that is the one I could best judge of in the single alteration which I ventured to make, because in it I was assisted by the upper lighthouse tower rising as a scale between me, when standing in the lower light lanthorn, and the horizon. But it is also evident that, as all the refraction in the lens of this lighthouse is in a vertical plane, (and none in the horizontal plane,) that, what may be done for one azimuth, may be done for all by adjustment, provided the various parts of the lens, in different azimuthal directions accord with each other; i.e. if all the parts of the polyzonal lens are of proper dimensions and symmetrical, the adjustment for one part would be equally effective for the whole. We had not time to go round to the N.E. but I shall be very glad to know what the appearances of the lights are from 5 to 15 miles or more outwards, in that direction.

On the whole it seemed to me that this lamp was very much superior to the French lamp of the night before; and judging from the direction in which we last left the lighthouse, as much so as I should have expected. I must state that the consumption of oil and oxygen on that night was greater, (according to Mr. Hall’s report,) than on the day of comparative experiment; being at the rate of 9.35 pints of oil and 114.9 cubic feet of oxygen for the 12 hours;- and also, as before said, that we had the additional effect of the corrected plane mirrors.

I am tempted to venture a comparison here of the Bude with the upper or reflector lights. As we pass round the horizon we observe more or less light from the system of lights, according as we are near to the focal line of a reflector or to that which is intermediate between two such lines; and, judging from the difference in the degree of illumination, and the period of time during which the one or the other degree continues, I should think it would require 36 reflectors in the circle to bring the whole of it up to the maximum degree exhibited in the present state of things. But if the effect towards the S.W may be taken as an indication of what may be done with the Bude lamp in every direction, then it would seem possible that the latter might be made equal by adjustment to twice or thrice 36 Argand lamps in reflectors. I do not say it will be so, because, I have not examined the adjustment and other circumstances of the reflector lamp nor have I been able to go sufficiently into the Dioptric arrangement; but it would not surprise me if such a result were finally wrought out.

As we receded considerably from the lighthouses, and approached the horizon either in a S.E or S.W direction, the difference between the lights became less, and at last the two were nearly alike. Now this may have been the consequence of the adjustment; i.e. my alteration may have brought the maximum rays which were before going above the horizon, a little below it, and so the Bude lamp may have been employed in most effectively illuminating the space between the horizon and the shore; whilst the reflectors in the upper lighthouse were so adjusted as to illuminate that space less, but the horizon more.

But in relation to this matter, before I went to sea on Friday night, I examined the light of the lower lighthouse from the upper lighthouse at different heights. A straight line drawn from the lanthorn of the lower lighthouse to the horizon cuts the upper lighthouse at somewhere about 38 or 40 feet from the top, and that was the line along which I wished to direct the rays of the Bude lamp. Then on Friday night, after the adjustment and before going to sea, on looking at the lower light from the base of the upper lighthouse there was a certain degree of light in the visible lamp flame; on ascending, this evidently and greatly increased and was best at about 3/5 of the way up; it was also much better above than below that part. Hence I had every reason to believe that the principal mass of rays was shining in that direction on to and above the horizon. Now this is just the direction (the S.W.) in which the Bude light produced its best effect; is that in which the adjustment is most certain; and yet is one in which the lights approximated in character and strength when the observer was on the horizon.

I cannot help thinking that the different altitudes at which the lights are placed may have a great deal to do with this apparent approximation in strength of the lights when at a considerable distance. The upper light is 92 feet above high water mark; the lower light is 57 feet above the same mark; and the eye of the observer is usually taken at 10 feet above the same level. According to the usual and correct rule the lower light would be run down or disappear to the eye at 13.11 miles distance and the upper light at 15.61 miles distance; (no account being taken of the effects of refraction;) and the distance to run between losing the two lights would be 2½ miles if the lighthouses were together, but which in the present case has to be increased by 4300 feet or thereabouts, because when off Harwich the lights are end on:- the distance therefore required to produce this effect is theoretically, 3⅓ miles.* Hence when an observer is receding from the two lighthouses the ray which comes to his eye from the Bude or French lamp is always passing through a lower stratum of the air than that from the upper light; and at the distance of 12 or 13 miles it is passing for a great part of it’s course through that air which is lying close upon the surface of the waters, whilst the ray from the upper lighthouse is passing a considerable distance above it. Now I do not know from observation whether this would make a difference, but I should expect that in many cases, if not in every case, it would; and that the effect would be for equal long distances a greater extinction of the rays of the lower light than of the upper. The whole question is, whether, if the French or Bude lamp were in the high light and the Argand reflectors in the low light, the former would not rise in comparative intensity, and the latter fall, when seen from the horizon, in place of the reverse effect which now occurs? or whether, if both the upper and lower houses had lights of equal intensities, (as Argands in reflectors) those in the lower house would not at equal considerable distances lose more of their light than the upper.

I understand that on proceeding for London, after the upper light is visible two miles more of progress brings the lower light into sight. This is in favour of the view I have given above; for if 3⅓ miles is the true interval of their points of appearance or dis-appearance, then it is probably because of the greater relative strength of the lower light that it’s limit of disappearance approaches so much to the limit of the upper; the upper or weaker light giving way much more rapidly to the deteriorating effect of the vicinity of the water when it comes to pass near it, than the lower light had done before.

I was honored with the company of Capt: Hayman⁴ in this trip and am indebted to him for kindness more than I can properly thank him for. I have not referred to his judgment regarding the lights when seen from different points of view for I thought it would, under the circumstances be impertinence in me to attempt to state it; but I may be allowed to hope that it does not, as to the facts, differ materially from my own.

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Production of Oxygen. Every part of the gas apparatus appears to work well and regularly. The retort looks sound, and will probably last, not merely as long as those in the Pay Office, but for a much longer period. The lighthouse keeper prepares the oxygen and obtains the full amount from the Manganese. The Gas-meter, which indicates the nightly consumption, is not properly graduated, and once or twice mistakes in the estimate have been the consequence.

The Lamp. I have on a former occasion referred to the loss at the lower part of the lens of the back light or that of the jets at the opposite side of the lamp. It is greater in proportion with the Bude than with the French lamp; because the Bude lamp has a larger diameter; because the bright part of its flame is shorter; and because it does not gather over towards the middle as the flame of the French lamp does. Should the Trinity Board resolve that the present performance of the Bude lamp is such as to justify further endeavours to improve it, I think it very probable that alterations for the better might be made in this respect; but it would be improper to alter anything whilst the lamp is still in Mr. Gurney’s hands.

I feel myself bound to state that the construction of the lamp is a more delicate matter than that of the Argand lamp. This is a fact sufficiently illustrated by the difference between the present and the former lamps. A great deal appears to depend upon the careful formation of the oxygen jets.

Irregularity of illumination. If an observer were to travel at some distance round the Orfordness lighthouses crossing the outward course of the rays, he would observe variations in the illumination, the illuminating power not being equal in all directions. In the upper lighthouse these occur in various ways; thus, as before said, the focal line of each reflector is most usually a line of maximum light whilst the intermediate lines are lines of minimum light. A little error in the centering, or in the working of the reflectors, or in their inclination will not only displace these lines more or less but, by combination also form new ones and thus, I believe, is produced the irregularity in the direction and extent of the more and less illuminated parts of the space around the reflector lighthouse.

There are also spaces of maximum and minimum illumination around the lower lighthouse but these occur in a different way. The Polyzonal lens refracts only in a vertical direction; if it were perfect therefore in every part there should be no lighter (or darker) part in one direction on a horizontal plane, more than in another. But the lens is held together partly by certain vertical brass bars, of which there are five in 180° or half the horizon, and these being 37 inches from the light are each 1⅜ inches wide. These offer so many opaque substances to the light, and in the direction in which they occur, shade full a third of the flame from the Sailor. For this there is no compensation except the irregularity of workmanship and of the quality of the glass in the lens; for there is no intentional refraction or bending of the rays in a horizontal direction.

The vertical window bars act in a similar manner of these there are 12 in the 180°. Six of them are 1¼ inches wide, and six are 2 inches wide; their distance from the lamp is 70 inches. Any one of these exerts its full shading power over the lamp in the direction in which it is placed. It so happens that the bars of the lens, and those of the window do not coincide, except in one place in the half circle, so that their whole shading effect is exerted, to the diminution of the light. As an illustration of the effect of these circumstances, I may refer to the Shipwash Light; it is directly in the line of focus of one of the reflectors of the upper lighthouse, but on observing it from the lower lighthouse, I found that a lens bar of 1⅜ inches width and a window bar of 1¼ inches both came between it and the lamp; so hiding 2⅝ inches of a lamp the flame of which was only 3⅞ inches wide. This occultation is compensated for only by the irregularity of the Glass and errors of workmanship.

In the erection of new lighthouses on the Dioptric principle it would be well to make the panes of glass as large as is consistent with a proper degree of strength; and if possible to make the upright bars of the window and of the lens coincide. The panes of glass in the low Lighthouse are 17 inches square.

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I cannot resist making a further remark or two on the two modes of illumination at the Orfordness Lighthouses. That in the upper Lighthouse has no doubt the advantage in the simplicity of the arrangement and in the easy care of the lamps and mirrors; and though I think that an intelligent man may easily give all the care that is required in a Bude Light, yet I also think that the care required to obtain the full amount of light from the oil and oxygen consumed, is more than with the Argand or with the French lamp.

The Maladjustment of the Mirrors in the lower Lighthouse shews, I think, that in all possible cases the final adjustment should not be by measure alone but also by trial; and I purpose having a simple instrument constructed which shall assist in attaining this object in many cases. The plane mirrors in the lower lighthouse may easily be adjusted, since each has separate adjustments; but with the lens part this is a more serious matter. It consists of innumerable parts, which are permanently fixed together, and also in their place in the lanthorn, so as to allow of no further alteration. Now, not to speak of the smaller portions, if the great parts are not accurately adjusted to each other at the time of fixing, it then becomes impossible to arrange a central lamp so as to produce its proper effect all round. I cannot positively say it is the case, but I have an impression that the two parts of the lens to the S.W. and N.E are not consistent in their action, for by observation both with the French and Bude lights, it appeared to me that the refracted beams in these directions were not in a straight horizontal line; or, in other words, that when the great mass of light in a S.W. direction was refracted horizontally that in the N.E. direction was refracted downwards a little; or that when that to the N.E was by adjustment of the lamp rendered horizontal that to the S.W was thrown above the horizon. Such an effect as this may be dependant either on the fixing of the individual pieces of glass in their respective frames or in the fitting of the frames themselves (of which there are five in the 180°) in the lanthorn, and it cannot be corrected by any change of place or adjustment of the lamp. In the upper lighthouse system each lamp and reflector can be adjusted by itself; and in that respect has the superiority.

Before concluding these remarks on adjustment, and this report, I will observe that theoretically the effect of the atmosphere is such as to bend the rays from an elevated lighthouse downwards; so that a ray issuing from a high lighthouse in a direct line towards the sea horizon would not reach that point, but would be depressed and come down to the water before it had travelled so far. Whether the effect is of any practical consequence or no, I have not had the opportunity of observing but it may require to be considered in the adjustment of the lights⁵.

I have the honor to be | My Lords and Gentlemen | Your obedient and humble Servant, | (signed) M. Faraday

Royal Institution | 12 August 1839.

*This is for the state of high water; at low water, the intervening distance would be less. I do not know, but believe it was approaching time of high water on Friday night (or Saturday morning rather) when we ran the lights down on coming home.