« PreviousContinue »
The details of this investigation might perhaps, with propriety, have been decently buried out of sight, and the result merely laid before the reader. It seems, however, not undesirable that, the matter having been fairly sifted and settled, there should be a record of the evidence on which their respective shares in the work fall to be assigned to Colby and Drummond. And such a record is, perhaps, the more desirable, that in reliance on the passage in “The Familiar Lectures” the whole merit of the compensation bars has been recently, in a work of reference, assigned to Mr Drummond."*
We come now to the devices employed for testing the accuracy of the measurement as it went along.
It would be out of place in a work which it is hoped may have interest for the general reader, to enter into the particulars of the contrivances for securing the levelness and straightness of the base-line. Everything that ingenuity could devise was done to attain these ends. A novel feature in the measurement, however, and one which falls to be noticed, was the employment of triangulation for the verification and extension of the base. This was a device of Drummond's to prevent the possibility of the measurement of a single bar or microscope being omitted in the daily record, as well as to ascertain the probable error which might be involved by adding a portion to the base on its own prolongation by triangulation. Remeasurements were also made whenever there was reason to apprehend that there had been any accidental derangement of the apparatus. Sir John Herschel and Mr Babbage were at one of these remeasurements, when the coincidence of the two measurements was so close as to be surprising
* “Chambers' Encyclopædia,” Art. TRIANGULATION.
even to men accustomed to deal with the smallest quantities measurable by the micrometer. * An admirable pencil sketch (copied in the annexed woodcut), taken by Sir John on the spot, and now in his possession, shows the process of measurement, the six bars in position under their protecting tents, and the officer in charge engaged in observing the compensation microscope. It is the sketch alluded to in the following letter—the only account of the measurement, written by Drummond, that has been preserved, and which has been put into my hands by Sir John Herschel, to whom it was addressed :
' MANCHESTER, Nov. 25, 1833. “MY DEAR SIR,-In consequence of the wandering life I am at present leading, and of some mistake at a country post-office, your lettter of the 28th ult. reached me only a few days ago. Not having a single paper or document of any kind to refer to, I must answer your questions about the Irish base-line rather from the impression which some previous inquiry has left upon my mind, than from any very distinct recollection of the circumstances which have produced it. The distance is about 74th miles, and the error I believe not to exceed 2 inches. I shall endeavour, to the best of my recollection, to state to you some of the grounds of my belief. The line is intersected by the river Roe, not deep, except for a few yards, but having a width of 480 feet. We looked forward to the crossing of this river with some degree of apprehension; it was necessary to drive piles the whole way across to support the bars, an operation of some difficulty and expense; and although every precaution was resorted to in order to render them steady, it was not sufficient to prevent the tremulous motion produced by the current. It was therefore considered indispensable that this portion should be measured twice. On the first occasion
* It appears from the official account that this visit was paid on the 13th September 1827, a few days after the measurement commenced.
SKETCH SHOWING THE MODE OF PROCEEDING IN MEASURING THE LOUGH FOYLE BASE.
(The Original by Sir J. F. W. HERSCHEL, Bart.)
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 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
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