I know to have been made with excellent instruments, and, it may be hoped, with due care. Until such a publication takes place, we must be permitted to withhold our final assent from a conclusion which is in contradiction to observations on at least three other glaciers, and to former observations on the same one. Should it prove correct in the particular case of the Glacier of the Aar, I suspect that it can only arise from a curious balance of two opposing influences occasioned by the peculiar circumstances of restraint under which that glacier moves, and probably may apply to only a very limited portion of its surface. But I repeat, we must wait for the observations. The public cannot but receive with distrust, reports of conclusions drawn from unpublished observations so repeatedly contradicting one another, as those which have been furnished by observers on this same Glacier of the Aar. Not to go farther back, within four years we have had four positive statements of fact said to have been deduced from observation, three of which are irreconcileable. First, we were told that the glacier was absolutely quiescent in winter, and moved onwards during the summer months only. It was next admitted, that in winter there is also motion, only the summer motion is greatly in excess. Next year (1844) we were told that the variation of velocity is not confined to summer and winter, but is extended to every variety of meteorological condition, in fact, is left on the basis on which I had already placed it (as regarded the Mer de Glace of Chamouni) two years previously. In the middle of summer, but during cold and snowy weather, which lasted nine days, the velocity of the Glacier of the Aar fell below the mean velocity of the same point for the entire year; whilst during the succeeding sixteen days of fine weather, the daily average increased in amount by exactly one-half, and rose con * Agassiz, 1842.-" Ce que je puis annoncer positivement des à présent, c'est que le glacier est immobile en hiver." Letter to M. Arago, dated from the Glacier of the Aar, Comptes Rendus, 8th August 1842. Compare Edin. Phil. Jour., 1842, vol. xxxiii., p. 253, 254. † Agassiz, 1843.-"Le mouvement est beaucoup plus accéléré en été qu'en hiver." Bulletin de la Société des Sciences Naturelle de Neufchâtel, 8th Nov. 1843. 1847.] OBSERVATIONS ON THE GLACIER OF THE AAR. 209 siderably above the annual average.* Now (1846) we are desired to consider these opinions and deductions to be erroneous, as well as the measured distances ascertained by M. Wild, who has hitherto been believed to be a competent surveyor, and who had found the advance of the points of his triangulation on the glacier to be more than one-half more rapid during summer than in the remainder of the year. The opinion of MM. Dolfuss, Martins, and Collomb, is, that seasons and weather make no difference whatever on the motions of the glacier of the Aar! From these conflicting results, it is plain that there have been reckless assertions made, and also observations unworthy of confidence. It is very much to be desired, for the credit of all the parties who have had a share in the experiments on the glacier of the Aar, that the source of these discordances should be investigated. I remain, etc. EDINBURGH, 12th March 1847. [The observations on the motion of the Mer de Glace, referred to in the preceding letter, and which were printed at its close in the Edinburgh Philosophical Journal, being textually taken from the paper in the Philosophical Transactions for 1846, and already printed at page 125, etc. of the present volume, are of course omitted.] * Comptes Rendus, 9th Dec. 1844, p. 1301.-"L'avancement [journalier] était loin d'étre uniforme, il variait considérablement suivant les conditions atmospheriques." Compare Ninth Letter [p. 73 above]. + Bull. de la Soc. de Neufchâtel, No. 1.-From the results there mentioned, I give the comparative motion of seven points of the glacier during fifty-seven days of summer, and for the same length of time taken from the average of the remainder XVIII. FIFTEENTH LETTER ON GLACIERS. Addressed to the Rev. DR. WHEWELL.* OBSERVATIONS ON THE ANALOGIES DERIVED FROM MUD-SLIDES ON A LARGE SCALE, AND FROM SOME PROCESSES IN THE ARTS IN FAVOUR OF THE VISCOUS THEORY OF GLACIERS. Land slips as observed by M. Collin-Mr. Milward on the Phenomena of a large Mud-Slide at Malta, and on the Cause of the Dirt-Bands and Wrinkles of Glaciers-Connection with Frontal Dip-Surfaces of Detrusion in Iron Turnings, attended with Vertical Accumulation of Material. My dear Sir-It is considerably more than a year since you did me the favour to communicate to me the interesting drawing and remarks by your friend Mr. Milward, on a mudslide on a large scale, which had come under his observation at Malta, and which led him to notice some interesting analogies with the structure of glaciers. Again, last August, you communicated some farther reflections and observations by Mr. Milward, and you invited me to send any remarks on the same subject which occurred to me, to be communicated, along with Mr. Milward's papers, to the meeting of the British Association. I sent you, on the 11th of August, a letter, the chief parts of which I shall embody in this one, but which was not read at Swansea, in consequence of the pressure of business in the Geological Section, which barely admitted (as I afterwards heard) of Mr. Milward's papers being read, and consequently no discussion took place. Since that time, Mr. Milward, before returning to Malta, was kind enough to place his papers at my disposal, which I then offered to Professor Jameson for publication in his Journal, which he accepted, and now allows me to add my remarks on the same subject, which I address to you, as having been the introducer of Mr. Milward's facts, and as having first desired my opinion with regard to them. The phenomena presented by mud-slides on a large scale, are not now studied quite for the first time. About two * Edinburgh New Philosophical Journal for January 1849. |