decay. The experience of the world has taught us that they fall much faster than they rise. Without due encouragement astronomical science may gradually decline; and it is of so much importance to the honour and the interest of our nation to support it, that I scarce think such a memorial as would emanate from your pen could prove altogether ineffectual." Since the time when Captain Colby thus forcibly expressed his opinions on this interesting subject, the multiplication of working observatories, public and private, at home and abroad, and the many important discoveries which have been the result of such continuous and wide-spread observations, have at once, as it were, responded to his appeal, and justified its wisdom. In the earlier period of his connection with the Survey, Captain Colby had usually established his residence, when not in the field, as near as possible to that of General Mudge, but his immediate presence at head-quarters became desirable when the work of reducing the actual surveys to the scale required for engraving, which necessarily increased proportionately to the number of surveyors employed in the field, had so far augmented as to render constant supervision indispensable, and he had, therefore, taken possession of apartments at the Drawing Room, or Ordnance Map Office, at the Tower. Surveyors belonging to the corps of Surveyors and Draughtsmen were now employed in various parts of the country, by permission of the Chief Engineer (Inspector-General of Fortifications), who was the official head of the corps, and they received, besides their military pay as warrant officers, 32s. 6d. per square mile for all the work they surveyed and plotted on a scale of two inches to the mile. These plots were reduced at the Map Office to plans on a scale of one inch to a mile, and traces from them prepared for the use of the engravers. It will be readily believed that the superintendence of all these operations, simultaneously proceeding at so many different places, some in the Map Office, some at various parts in the country, combined with the personal direction of the triangulation in Scotland, was enough to overpower the mental energies and the physical powers even of a Colby; and in 1816, therefore, at the recommendation of General Mudge, his son, Lieutenant Richard Mudge, R.E. (now Lieutenant-Colonel Mudge), was attached to the survey, and stationed in the country, as nearly as possible in the centre of the surveying parties, of which he had the immediate superintendence from that period for several years. This partial relief enabled Captain Colby to visit nine stations in Scotland during the year 1816, without the harassing distraction of mind which had hitherto attended the complicated charge of such distant works. In 1817, also, Captain Colby was for a long time in Scotland, and in that year measured, with the assistance of Mr. Gardner, the single base line of that kingdom, on Belhelvie Sands, near Aberdeen. The measurement of this base occupied from May 5th to June 6th, and Ramsden's steel chain was again the instrument used. Its length, when compared with the unit Ordnance standard bar O, is found to be 26516-66 feet, and the length, as deduced from the Lough Foyle base, is 26518.99 feet, showing a difference of about 2 in five miles. Some of the old bases have afforded a better and some a worse comparison than the above; for example— Hounslow Heath base, with glass rods, when reduced to the Ordnance stand 27405.83 Deduced by computation from Lough A result which may be considered unparalleled in Misterton Carr, steel chains, 1801. Computed from Lough Foyle 2634372 . 26350.76 The difference here is considerable, and has been partly accounted for by the unfavourable ground; but it must be remembered that the comparison before quoted between this base as measured, and its length as deduced from other bases, was not so unfavourable as to render this explanation quite satisfactory F Rhyddlan Marsh, steel chains, in 1806 . . 24515.2 Computed from Lough Foyle. . 24518.2 It is impossible not to see in these facts proofs at once of the great care applied to the measurement of the first bases, of the precision attainable by General Colby's compensation bars, and even by the steel chains under due restriction, and of the superiority of the triangulation by which they have been linked together. This high character, indeed, was always maintained by the British survey, even when the results, which have been pointed out, were the subject of so much discussion. M. Arago, when resuming, in a letter to Colonel Mudge, the subject of a connexion between the English, French, and Spanish measurements, says: "If this proposition should appear reasonable, it will afford me the further advantage of manifesting to you the confidence with which your operations have inspired those scientific men who are best able to appreciate them;" and Schumacher was so satisfied of the excellency of Ramsden's sector, that on his recommendation the King of Denmark requested the loan of it for the celestial observations of the Danish arc: "The illustrious president of the Royal Society has informed me that you (Colonel Mudge) are about to re-examine your celestial arc in the following summer, and that you approve of lending the sector to the King for the determination, also, of our celestial arc. May I beg you to inform me whether it will be necessary that I should apply to any other person than to you, who have used the instrument in 6 the beautiful work you have just terminated." It is remarkable that in this letter (1816) Schumacher anticipates what had now been satisfactorily proved, namely, that a considerable deflection of the plumbline had occurred at Dunnose: "I have not seen," he says, "what Dr. Gregory has published on the subject, and I know only your observations by what has been printed in the Philosophical Transactions,' but from that I feel assured that the new observations will only confirm the old. You, who know the ground, are best able to judge where local attractions are most probable; but, on my part, I must confess that I should suspect them rather at Dunnose than at Arbury Hill. And further, it seems to me that in any hypothesis it would be very difficult to establish a perfectly regular figure for the earth. Such an hypothesis is, indeed, the least probable of any; and we ought not, therefore, to condemn observations because they tend to destroy an hypothesis which has so many chances against it." It may be here observed that the instruments used on the Continent and in England for determining zenith distances were very different; as the sector, improved by Ramsden (several had been made of very large dimensions, by Sisson, at a very early date for example, that of Pisa, 1765, and Dr. Maskelyne's, both of ten feet radius), continued the favourite instrument with the English geodesists, whilst the repeating circle of Borda has been generally adopted by French and other continental observers. The principle of the repeating circle, so far |