« PreviousContinue »
The original light apparatus had been designed for the Survey. Portability, rather than economy, had been studied in its construction. The ball of lime-from which when intensely ignited the light was derived-was heated by means of a stream of oxygen directed through a flame of spirit of wine. This source of heat was expensive, and, in considering the changes now to be made, economy was a primary object. For the alcohol Drummond substituted hydrogen gas, which proved not only much more economical, but productive of a considerable increase of brilliancy in the light.
The new apparatus was exceedingly ingenious. The oxygen and hydrogen gases, proceeding from separate gasometers, arrive at a small chamber, where they are made to mix. Into this chamber the oxygen gas is projected horizontally through a series of very small apertures, the hydrogen gas rising into it vertically through a series of similar apertures. From this the united
gases pass through two or three pieces of wire gauze, and, being thoroughly mixed, issue through two jets against the lime ball in the focus of the reflector. To prevent the wasting of the ball opposite the two jets, and at the same time to diffuse the heat more equally, the ball is made by the apparatus to revolve once in a minute. Notwithstanding this movement, the heat cuts a deep grove in the ball, and it becomes necessary to replace it by a new one every three-quarters of an hour. As it would be unsafe in a lighthouse to intrust the replacing of the ball to an attendant, the apparatus is so contrived that it effects the replacement itself. The number of balls required to maintain the light for any time are placed on a wire passing through the focus of the reflector; the ball in the focus drops the moment it is sufficiently worn, and its place is instantly taken by another—the next above it on the wire—which two minutes before has fallen into a position to be gradually heated for doing duty in the focus.*
From a letter dated January 16, 1830, written to his sister, it appears that the Duke of Clarence, who a few months later became King William IV., had been expected to witness the experiments in his character of Master of the Trinity Corporation ; and that an experiment which had been made with the lime-light, before the apparatus had been brought into perfect working order, had resulted in an explosion. Miss Drummond had cautioned her brother not to enter upon the experiments till he had completed his preparations.
“ LONDON, January 16, 1830. “MY DEAR ELIZA,- A week, more than a week, has passed since I ought, and since I intended, to have answered your kind, kind letter ; but every day and every evening has brought such constant occupation that I positively have not had time.
“The consequence, no doubt, has been many conjectures, and much exercise to my dear mother, if the bell rang about posttime. Now, what have been your conjectures ? Another explosion, perhaps, and the heir-presumptive, along with all my beautiful apparatus, sent to the upper, or perhaps the under regions; or everything gone off well, and the Duke extremely
* This description is an abridgment from Mr Drummond's paper in the “ Philosophieal Transactions." There is an alternative contrivance for maintaining the constancy of the light in certain
“Wherever the light is required to be diffused equally around, the renewal of the lime may be effected still more easily, by using a cylinder instead of a ball, which being gradually raised while revolving, brings fresh portions in succession opposite the jets. In a reflector a cylinder occasions partial shadows at the top and bottom ; still, however, the simplicity and certainty with which it may be renewed, will probably entitle it to a preference even in this case." - Phil. Trans. 1830, p. 388.
delighted, expressed himself highly gratified, and intended conferring upon me some signal mark of his royal approbation! Well, to keep you in suspense no longer. The Duke was not present; he was unwell, and unable to leave his house. We were all prepared, for the messenger did not arrive till the last moment. The next Board day, when he is expected, is the 5th February. Meanwhile we proceed with the experiments, and it is with them that I have been so much engaged this week. But this is Saturday evening, an evening of repose and enjoyment, and I have taken advantage of it to discharge my debt to you. I was grieved to hear of more colds and plaisters, and I fear much that this fierce weather does not agree with you.
Do you ride? How is the pony ? Has John recovered, and has he been laying down the law? I think you might manage among you to write a little oftener. There are some long gaps in our correspondence, and some long intervals during which I hear nothing of you.
Almost all my acquaintances have been ill more or less. I have great reason to be thankful that I have kept so well; indeed, notwithstanding all my work, I am in rude health, sleep but one sleep, and no palpitation. All the advice you gave me in your letter I acknowledge to be excellent, yet the exhibition was unavoidable, and so was the explosion. But I think they have got over it; if not, I will tell them the first time I have an opportunity of making a speech, that if I had been making an experiment before men unacquainted with the peculiar nature of such experiments, I should have declined proceeding under such circumstances; but before enlightened and intelligent men, whose indulgence and partiality I had more than once experienced, I could have no hesitation in trying even a first experiment, deeming it the best compliment I could pay them to show them the apparatus under the most disadvantageous circumstances.
My best and kindest love to you all at home. Adieu, my dearest Eliza, and believe me your ever affectionate brother,
T. DRUMMOND." The Duke of Clarence was present, and highly gratified, when the lights were exhibited at the Trinity House on the 5th of March ; after the exhibition he
presided at a dinner of the Corporation, to which Drummond went as a guest. By the month of May 1830 the scene of the experiments was transferred to the lighthouse at Purfleet. The light was making a sensation again, many of the most distinguished men in London going to witness its brilliant effects. Though Drummond did not oppose the experiments for testing the efficacy of the light at a distance, as that might have been impolitic, his view of them, expressed to his mother, was, that they were “mere trifling, and waste of time.” They were spreading his reputation, however, and every day bringing him new friends. By this time, six of his lights had been ordered by the East India Company, at the cost of L.100 a-piece. He was astonished at, but rather regretted, the Company's liberality, considering the hands into which the lights would fall. “ I am busy,” he says, “preparing an account of our operations in Ireland, to be delivered as a lecture on Friday at the Royal Institution.”* Friday was also to see the last of the experiments on the lights. “ I will then present a report to the Trinity House, and they will decide what further steps should be taken. I am writing a paper on the experiments for the Royal Society.”+
This paper has been already referred to. It opens with a rapid history of methods of illuminating lighthouses, and account of those in use in the best lighthouses of Great Britain and France ; a description, accompanied by illustrative plates, of the new apparatus
* Letter to Mrs Drummond, 9th May 1830.
+ Drummond was elected a Fellow of the Royal Astronomical Society, May 14, 1830. He was proposed in March of the same year. In a letter belonging to that month, he says, “ I am doing all I can to render myself an efficient member of the Astronomical.”
for exhibiting the Drummond light is next given, and is followed by a summary of the results of the Trinity House and Purfleet experiments.
The first set of the experiments were on the illuminating powers of the different lights, independently of the lenses or reflectors with which they are generally used. In these the method of shadows, and that of equally illuminated surfaces, both dependent on the same principle, but requiring different instruments, were employed. The standard used was an Argand lamp, } inch in diameter, supplied with the finest spermaceti oil, and capable of supporting a flame 14 inch in height. It was found that the light emitted by the French lamp, a large Argand, with four concentric wicks, was equal to ten standards ; while that emitted by a lime-ball only
of an inch in diameter, heated by two jets, was equal to thirteen standards. The next set of experiments respected the intensity or intrinsic brightness of the lights, the property on which their utility in lighthouses more immediately depends. The French lamp was found to be four times, while the lime-light was 264 times as intense as the standard ! These results,” says Mr Drummond, "were obtained by screening the lights, and then placing equal apertures opposite each, changing the apertures, and taking the mean to destroy the effect of any inaccuracy in size. The intensity of the lime-ball being, therefore, 264 times that of the Argand lamp, a single reflector, illuminated by the former, will be equal to 264 reflectors illuminated by the latter ; but the divergence of the reflected light, depending on the size of the luminous body in the focus, will be smaller with the ball than with the lamp, in the proportion of about 3 to 8; hence, in such a lighthouse as that of Beachy Head, 8 reflectors may be