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we commenced at low water, having the advantage of shallow water, but the disadvantage of a stronger current; on the second we began at high water, in order to vary the circumstances as much as possible. The difference was 3 inch between the two measurements. Now this was beyond comparison the most difficult and troublsome part of the whole line. Again, at the commencement of the operation., before we had become expert at the use of the apparatus, when we proceeded very slowly, it was thought desirable to remeasure the first three or four hundred feet. You may recollect that you and Babbage arrived at the very moment when we were concluding this experiment, and that there was scarcely any perceptible difference between the terminating dots. In this case the ground was firm and good. This was indeed its general characteristic; but there were certainly portions boggy and elastic, which gave us considerable trouble. In passing over these, we of course resorted to every precaution we could devise to ensure accuracy; all the intersections were made at once, and several times repeated ; and weights were placed so as to preserve each terminating point of a series under similar circumstances, whether it formed the first or last of a series. No portion of this ground was, however, twice measured. In addition to these repetitions and precautions, points were preserved with great care at different parts of the line, in order to furnish the means of trying one portion against another, by means of a triangulation; and the results of many trials furnish perhaps the most satisfactory proofs of the accuracy of the measurement.

" The series of triangles [exhibited in the woodcut annexed, copied from Yolland's “ Account”) is spread like net-work on both sides of the base; and the difference of 1, 2, and very rarely :3 of a foot, on large distances, are pretty good proofs that our errors cannot be great. From some rough calculations made at the time, I came to the conclusion already stated. I meant to have gone through these more accurately, though there are many circumstances which it is difficult to submit to calculation. Other occupations have intervened for a time, but I have not abandoned my intention, and shall communicate to you the result. This, I think, is all the information I can give in answer to your first question.

“ With respect to your second question, as to the base on Hounslow, the measurement by General Mudge is the test of its accuracy; and I must refer you to the account of the survey rather than venture to make any statement from memory. The difference was, however, very small. Indeed, it is somewhat mortifying to find that, with an apparatus as superior to

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the wooden rods of Boscovich and Bonquer, as a microscope micrometer is to a pair of carpenter's compasses, we have not attained to much greater accuracy than they themselves claim. If they are right in their estimation, or correct in their statements, our labours have been in vain.

“The extremities of the base at Hounslow were marked with guns (very useful for such purposes), and, during the radical riots several years ago, orders having been issued to remove all the guns on martello towers, in the batteries, or wherever they were to be found, the indiscriminating dogs laid hands on the sacred pedestals, and conveyed them to an ordnance store! I believe, however, that beneath the guns oaken piles were driven, and these it would not be difficult to find. The marks on Hanger Hill Tower and Shooter's Hill Tower are preserved. With respect to the extremities of the Irish base, the most satisfactory precautions have been taken to secure them. The bases, about 3 feet under the surface, consist of four slabs, about 3 inches thick, and 7 feet long by 3} broad. These are laid transversely, while longitudinally two blocks of compact sandstone, weighing about (I think) 17 or 18 cwt., and being somewhere about 4 feet square, are placed above them the whole firmly cemented with Roman cement. The surface is level with the surface of the ground, and the soil is very firm and secure. They were placed several weeks before they were used, to get rid of any little settlement to which they might be liable. The ground has, however, been purchased, and enclosed by the Ordnance.

“ There is little to attract attention, and there is sufficient strength to resist any ordinary attempts of the country people to remove them, supposing them to be so inclined, which I believe they are not. Nothing but the mandate of Captain Rock or Dan O'Connell would expose them to destruction. I am ashamed at having detained the sketch so long. It is safe, however, and shall be sent to you as soon as I return.

“ I have now endeavoured to answer your questions, very imperfectly I admit, but as well as my present situation and occupations will admit of my doing. If you require anything more, I shall be most happy to procure it on my return to town, which will probably be in a fortnight. At present I am engaged making some inquiries for Government, which occupy my whole time ; but I assure you

you do me a favour by affording me any opportunity of showing, though in a very humble way, the grateful sense I entertain of your kindness on many occasions.—Believe me, my dear sir, most truly yours, The actual measurement of the Lough Foyle base commenced on the 6th September 1827, and was continued without intermission till the 25th of October, when, in consequence of the advanced state of the season, it was deemed advisable to suspend the work till the following year. The measurement was resumed on the 7th July 1828, and continued till the 25th of the same month, when it was suspended, in consequence of the crops being on the ground over which the line lay. It was resumed after the harvest, on the 13th of September 1828, and completed on the 20th November of the same year.

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“ T. DRUMMOND."

5

CHAPTER IX.

THE ORDNANCE SURVEY; MR DRUMMOND'S HEALTH; THE

DRUMMOND LIGHT AND LIGHTHOUSES ; A VISIT TO THE KING ; LORD BROUGHAM.

AFTER the measurement of the Lough Foyle base, Mr Drummond remained for some time in Ireland, the head-quarters of the Survey being now in Dublin. In December 1828, and January 1829, he was engaged, along with Lieutenant Murphy, in comparing the measuring bars and the standards, and testing the compensation microscopes ; which work they resumed and completed in the following autumn. During a portion of the summer, they were occupied in observations for the triangulation in the vicinity of the base.*

The anxiety and exposure attending the operations in 1828 had seriously affected his health—the second, if not the third time, that a severe illness had sprung out of his devotion to the service. He suffered much, as in the winter of 1825, from palpitation and sleepless

* The observations made at Minearney by Messrs Drummond and Murphy in June 1829 are given in the Appendix to Yolland's “Account," where will also be found the results of the comparisons referred to, pp. 20, 21, and 22. In February, March, and April 1830, Mr Drummond was engaged in a similar set of comparisons.Ibid. pp. 15, 19. The last time he took part in experiments of this kind was in the spring of 1833, when, along with the Rev. R. Sheepshanks and Mr Simms, he was engaged in ascertaining the expansions of the Royal Society's brass scale.Ibid. p. 12.

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