under the surface. We long ago remarked, in speaking of the Trigonometrical Survey of England, that it would have been of great importance to have added to it a mineralogical survey, as the results of the latter might have thrown some light on the anomalies of the former. The same thing is suggested by the objects now under consideration. It would be extremely desirable also to have a vertical section in the direction of the meridian and of the perpendicular, at those places where observations for the latitude are made. This might afford a satisfactory solution of many difficulties which at present are sufficiently perplexing, and seem to increase just in proportion to the extent and accuracy of the observations." This admirable suggestion was subsequently advocated warmly, both by Colonel Mudge and Captain Colby, and the result was the appointment of Dr. M'Culloch to act as mineralogical surveyor, in connection with the survey of Scotland. Major Lambton had found the length of a degree in latitude 11° 59′ 54′′, equal to 60529 fathoms by one set of deductions, and by another 60449; and as the stations from which these results had been obtained were sufficiently remote from mountains to remove all suspicion of a disturbance from that cause, although the existence of some disturbance was undoubted, he attributed it to the effects of a bed of iron ore which had drawn the plummet northward at Dodagoontah, and southward at Bomasundrum, so as to give the celestial arc between Dodagoontah and Putchapoliam (to the south of both the other stations)

too little, and that between Putchapoliam and Bomasundrum too great—a case quite analogous to that of the English arc. "The anomalies," as Playfair goes on to observe, "which have occurred in the measures of degrees, and of which the appearances seem to increase in proportion as greater pains are taken to avoid inaccuracy, have naturally drawn the attention of mathematicians; and the question, what part of them is to be ascribed to error, and what to irregularity in the structure of the globe, has come, of course, to be considered. That a small part of them only can be ascribed to the former cause, is rendered probable by the very circumstance just stated; that they are not diminished, nay, that they even seem to be increased, by the care taken to avoid error. It seems clear from that consideration that the irregularities are in the object sought for, and are only brought more in sight by more microscopical observations, by the excellence of the instruments, the accuracy of the computations, and the extent of the lines measured." But though Playfair ascribed the fact that in Colonel Mudge's arc the degrees appear to increase on going from the north to the south to local irregularities in the direction of gravity, and not to instrumental errors, he characterises the paper of Don Rodriguez as being written with great knowledge of the subject, and full of sound mathematical reasoning, notwithstanding it takes the opposite view of ascribing the irregularities in the arc to errors of observation. “Don Rodri guez," he observes, if we mistake not, "is one of

two Spanish gentlemen who accompanied MM. Biot and Arago, and assisted in the operations by which the meridian that had been traced through France was extended to the southernmost of the Balearic Isles. He seems perfectly acquainted with the methods of calculation, and all the most recent improvements which respect the problem of the figure of the earth. We do not think that he has proved that the irregularities in this measurement arise from errors of observation; and we are of opinion, though their amount may now be more exactly estimated than before, that with regard to their cause the question rests precisely where it did. But though we are not convinced by Don Rodriguez, we must do him the justice to say, that his argument is fairly conducted, and that he has displayed great knowledge of the subject, and perfect familiarity with the best methods hitherto employed in the solution of this difficult problem. We have, therefore, observed with regret that this ingenious foreigner has been attacked in some of the English journals with a violence and asperity which the subject did not call for, and which his paper certainly did not authorise."

This just and candid criticism did not quite satisfy Colonel Mudge, who appeared to think that the Edinburgh Reviewer was unwilling to allow any other hand than his own "to wield the tomahawk" of criticism; but it is impossible, at the present day, not to concur with Playfair in thinking that Dr. Olinthus Gregory, to whom his remarks applied, was

quite unjustified in such a system of defence, and rather injured than assisted Colonel Mudge, by adopting it; and that Playfair wrote with no illfeeling towards the Survey or its managers, may be fairly deduced from his closing paragraph:-"It is with pleasure, therefore, that we see a meridian, which has been extended from the shores of the British Channel along the west side of England, viz., the meridian of Delamere, now produced into Scotland, where it falls on the east side of the island, and is about to be continued till it intersects the shores of the Murray Firth, on the Northern Ocean. The combined arcs in France and England will then extend nearly to twenty degrees; and in a few years we shall perhaps see the distance between the parallels of the Balearic and the Orkney Islands ascertained by actual mensuration. We believe that this important operation could not easily be in better hands than those in which it is actually placed; and, when it shall be completed, the British Army--in General Roy, R.A., and the officers who have succeeded him in the conduct of the English Survey, and in Major Lambton, whose works in the Peninsula of India we have been now treating of-will have the glory of doing more for the advancement of general science than has ever been performed by any other body of military men." A noble tribute of praise from such a man as Playfair, in comparison with which the ordinary articles of the public journals, whether written in defence or in attack, should have been regarded as mere dust in the balance.

Notwithstanding the little value attached by General Mudge to the remarks of Don Rodriguez, the subject was often present to his mind, and in May, 1814, he writes: "In reference to this matter, and still more in reference to the importance of satisfying the public mind, now very anxiously bent upon this point, I should have derived infinite satisfaction from the zenith sector being this year used instead of the circular instrument, and I am sure I cannot see why it should not be so used, for I should be in that case enabled the sooner to put up the zenith sector on Arbury Hill, which I have promised to do, and certainly will do, to vindicate the memory of Ramsden." At this time the triangulation had been pushed into Scotland, and so earnestly intent was General Mudge on the repetition of the zenith sector observations in England, that he does not appear to have approved, in the first instance, of the project of extending the arc of the meridian to Shetland, which most probably originated in Captain Colby. In the same letter he says, "Now, I beg of you most seriously to take into consideration that it is my wish, if you can accomplish it, to finish the sector work this year (1814). What use is there in your going to Shetland with the sector? I can see none. Certainly there is none, for the arc will be long enough if ended upon the remote part of Scotland, short of the isles, and by causing us so to do a finale would be given to the sector. Let us not prolong it unnecessarily, but give a speedy finale to the meridional operations; the public very anxiously look for it."