The refractions on the second and third arcs, I consider as most accurate, on account of the great distance between the stations; and also as more to be depended on, from the circumstance of the ray generally passing 300 feet above the ground. The fourth arc affords another instance of the refraction varying at one station, and remaining constant at the other. This, no doubt, was owing to the intervention of some partial stratum of air, nearer to Epwell than Broadway Beacon. The refractions, deduced from these contemporary observations are certainly inconclusive. The mean refractions, (neglecting the fourth arc) brought under one point of view, will be as follows. If the air had been in a quiescent state, previous to and also at the times when these observations were made, it might be expected that the differences of altitudes in the stations would be obtained, tolerably near the truth, barometrically. The remarks in the tables appertaining to the first and second arcs, shew that such opportunities offered; but those which belong to the third, prove the wind to have been fresh; and, as the space between the stations which constitute the extremities of that arc is 34 miles, nearly, it is not to be expected that a true result should be obtained. The differences of altitudes of the stations constituting the extremities of the two first arcs, obtained by means of the observed angles of elevation and depression, as well as from the heights of the mercury in the barometer, will be as follows. The little done on this subject, points out the necessity of doing more; it therefore remains with me to observe, that I shall lose no opportunity of employing the apparatus committed to my charge in the best and most diligent manner, both as relating to matters of refraction, and to all others connected with the Trigonometrical Survey. In the Introduction, page 2, it is stated that this Account would be comprized in three Sections, but it was afterwards thought more convenient to divide it into four." In Page 45, line penult. dele and Prittlewell. 14, for 1792, read 1772. 127, Printed by W. Bulmer and Co. 1.: ACCOUNT OF THE MEASUREMENT OF AN ARC OF THE MERIDIAN, EXTENDING FROM DUNNOSE IN THE ISLE OF WIGHT, TO CLIFTON IN COURSE OF THE OPERATIONS CARRIED ON FOR THE TRIGONOMETRICAL SURVEY OF ENGLAND, IN THE YEARS 1800, 1801, AND 1802. BY MAJOR WILLIAM MUDGE, OF THE ROYAL ARTILLERY, F. R. S. INTENDED AS A SECOND PART TO VOLUME II. ILLUSTRATED WITH EIGHT COPPER-PLATES. FROM THE PHILOSOPHICAL TRANSACTIONS. LONDON: PRINTED BY W. BULMER AND CO. CLEVELAND-ROW, ST. JAMES'S, FOR W. FADEN, GEOGRAPHER TO HIS MAJESTY, AND TO HIS ROYAL HIGHNESS THE PRINCE OF WALES, CHARING CROSS. |