:

indeed, as it is necessary to use plumb-lines or levels in the determination of altitudes-and on them depends the determination of latitudes-it cannot be expected that we shall attain to the same precision as can be readily secured in horizontal angles, or that we can obtain latitudes by ordinary or moveable instruments within two or three seconds." Baron Lindenau, of Gotha, expresses himself even more strongly "The zenith observations," he observes, "of M. Bessel, at Könisberg, which in themselves constitute so striking an accordance, give, nevertheless, declinations of the stars, which are less by 3.2" than those of Pond, Piazzi, and Oriani ;" and he adds, what has happened to Inghirami, in his measurement in Tuscany, has similarly occurred in all great geodesical operations. In France it was necessary, in order to reconcile the results to a spheroid, the flattening of which was to 300, suppose errors in the latitudes of Dunkirk, Evaux, Carcassone, and Montjouy, of 3·1′′, 5·8′′, 0·8′′, 3·6", and in England the results were still more discordant, as they represented an equatorial and not a polar flattening, and would have required a supposed error of 8" in the latitude, to reduce them to order."

1

In respect to the two other causes, whilst Baron Zach admits the probability of local attractions and disturbances of the plumb-line, rather from the unequal distribution and density of the matter below the surface, than from the action of hills projecting above it, he seems more disposed to ascribe the discrepancies experienced to irregularity in the form of

the earth itself, and quotes the opinion of La Place to that effect. Baron Lindenau, on the other hand, rejects such explanations, and ascribes the difficulty of reconciling geodesical latitudes and longitudes with those determined astronomically, entirely to the defects of astronomical observations, partly resulting from the imperfections of instruments and partly from the defects of physical astronomy. It is not intended to enter here into a detailed discussion of this most interesting question, but it seemed only due to the reputation both of British artists and British observers that the true nature of the difficulties they had experienced, as well as their generality, should be made apparent. Two of the causes of such difficulties which have been cited, namely, first, the improbability of determining latitudes astronomically to the utmost precision by moveable instruments; and, secondly, the effects of local terrestrial attraction, cannot be reasonably rejected, as more or less influencing geodesical results; but it seems difficult to ascribe such great differences as those observed in England and Tuscany to irregularity in the figure of the earth alone, although it is highly probable that there is a certain amount of such irregularity. The more the physical condition of the earth is investigated, the more evident does it become that the matter immediately below its surface varies greatly from place to place in its density and properties, a fact which is strikingly exhibited in respect to the progression of temperature on descending into the interior of the

earth. Whilst, for example, under ordinary circumstances, the temperature increases by about 1° Fah. in 55 feet, it was found, in a boring at Neuffen, in Wurtemberg, to amount to 1° in about nineteen feet, an indication of great internal heat, which could only be accounted for from the proximity of the boring to large masses of basalt, which still retained a very high temperature. Can it be doubted that such masses of very dense matter, only recently fused under enormous pressure, would be a powerful attractive force, and exercise a powerful disturbing action; and, further, that such masses may be subject even to change of position, and therefore may so vary in the amount of their action as to produce different results at different times? These considerations will assist the reader in appreciating the accuracy of observations and the diffculty in guarding them from the effects of disturbing causes. General Colby was concerned either as principal or assistant in all the sector observations on which the comparison between the terrestrial and celestial arcs had been or will be founded. With General Mudge he observed at Dunnose, Clifton Beacon, Arbury Hill, Delamere Forest, and Burleigh Moor, and, aided by Mr. Gardner, he conducted those of Kellie Law, Cowhithe, and Balta; and the zenith distances, taken at the same stations, with Airey's sector, were observed by Sergeant James Steel, of the Royal Sappers; so that the system of employing trained soldiers on this great work, introduced by General Colby, attained, during

his time, its fullest development, and it may be cited, as a proof how entirely he was above the feeling of petty jealousy, that he thus directed his own personal observations, the observations of his most able days, to be tested by Sergeant (then Corporal) Steel. But though, in the abstract, this result was most satisfactory, it may be doubted whether General Colby contemplated, at first, so great an extension of his system, and whether, in fact, it has not now been carried beyond the limits of sound policy. The survey was considered by General Morse, as pointed out in a preceding paragraph, a fitting school for young officers, and it subsequently became, when extended to Ireland, a real and practical school for both officers and soldiers in every branch of surveying. Such a school it ought still to be, but when the officers are entirely abstracted from that class of observations with which it is most desirable that they should be familiar, it may be feared that they will never become either practical surveyors or practical astronomers; and though it is most gratifying to render the Sappers and Miners equal to the performance of every description of duty, the officers of Engineers should, at least, keep pace with them, and be prepared either to direct or to participate in their scientific labours. The survey, indeed, should be a field of operation for those officers who have distinguished themselves at the Royal Military Academy or at Chatham by their acquirements, theoretical and practical, in geodesical science, and it should afford to them the

means and opportunity of perfecting themselves in every description of observation as well as of calculation.

In the volume of sector observations just published, Captain Yolland does not enter on a comparison between the geodesical and astronomical amplitudes of the great or complete meridional arcs, as he justly observes, that "such comparison will be more properly dealt with in discussing the results of the triangulation, when the question of the standards of measure employed at different times on the Ordnance Survey, and the elements for the figure of the earth used in computing the geodesical amplitudes, will have to be taken into consideration:" and adds, that the geodesical amplitudes he publishes must be considered only as approximative, "as at present they all relate to feet, in terms of the ordnance ten-feet iron standard bar O, which there is no doubt is too short as compared with the standard used for the trigonometrical survey in England by Major-Generals Roy, Mudge, and Colby, and whose results have been employed by the Astronomer Royal, Professor Bessel, and others, in the determination of the figure of the earth." This comparison of the standards, or, in other words, the determination of the actual unit of measure, was constantly present to the mind of General Colby, when projecting and maturing his arrangements for the Irish survey; and its importance is so manifest, that it may be hoped the experiments there commenced will be completed, and the comparative