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arrangement just described, the loop passes symmetrically through the simple knot, we have another six-crossing form, very much resembling the last, but which is essentially not amphicheiral. It is figured below in one of its forms-the others may be got by deformation and the schemes of the two kinds are appended for comparison.

Figs. 3 and 4
Fig. 5

ADBECADFECFB | A.
ADBECFDA ECFB | A.

It will be seen that the sole difference between the amphicheiral knot and that last figured, lies in the inversion of the positions of A and F in their even places in the scheme.

It appears, then, that none of these abbreviated methods, however useful as temporary aids to classification, can take the place of the scheme in fully describing the form of a knot and in measuring the amount of beknottedness in general. Especially is the scheme required in order to calculate the beknottedness in terms of the electromagnetic work. And this conclusion might, I think, have been inferred from the prominent part which the arrangement of the letters in the even places plays in determining the form of a knot; an arrangement of which only traces are left when we substitute the sign of the work at each junction for the letter attached to it, thus losing all control of the amount to be added or subtracted on account of the mere number of coils.

[Added Jan. 27th.]-Professor Clerk-Maxwell, to whom I sent some of the above results (and to whom, as well as to Sir W.

be understood of what may be called Knottiness, not of Beknottedness. For it has just been shown by a particular case that we can occasionally increase the degree of beknottedness, while diminishing knottiness, i.e., losing crossings by so altering their signs as to make some of them nugatory. The point thus raised, i.e., the distinction between Knottiness and Beknottedness, is a very troublesome and delicate one, and is obviously related to several of the difficulties pointed out in the present paper.

Thomson, I am indebted for various hints, usually in the especially valuable form of criticisms and reasons for doubt), has lately called my attention to a paper by Listing, of date 1847, part of which is devoted to the subject of knots. I have this morning obtained it from the Cambridge University Library, but have not yet thoroughly read it. As was to be expected, I find that the author has anticipated some of the contents of my papers; and he mentions at least one very curious fact, which I had thought possible, but had not observed, though it is very directly connected with one of the results of the present note. He virtually shows, by giving a particular case, that the method of deformation which I employ does not always give all possible forms of a completely knotted wire. I believe that this depends on the fact that a part of the scheme is amphicheiral. I propose to give the Society an account of Listing's method and results on the earliest opportunity.

3. Note on the Effect of Heat on Infusible Impalpable Powders. By Professor Tait.

Several years ago Professor Dewar gave me a specimen of silica in a state of exceedingly minute division, which had been produced in Dr Playfair's laboratory in the preparation of fluosilicic acid. I noticed at the time how much its great mobility is increased by heating-so that it behaves almost like a liquid. And I fancied that I observed close to the surface a thin stratum of what might by the same analogy be called a vapour; consisting of particles thrown up and falling back again, like the little drops thrown up at the surface of soda-water. I was inclined to ascribe these phenomena to heat directly-supposing that the particles were fine enough to behave, though in a very imperfect way, as the kinetic theory assumes the particles of a gas to behave. However this may be, the extreme mobility of such powders when heated on a platinum dish; and the fact, noticed by chemists, that a bath of calcined magnesia is capable of propagating waves when heated; seem to show that valuable results might be obtained by seeking for evidence of inter-diffusion as the result of experiments made by very long-continued heating of vessels containing fine silica and mag

nesia originally in separate strata. I have brought this before the Society in the hope that (as it can hardly be classed as a laboratory experiment) some of the Fellows, who may have access to a suitable furnace which is in activity the greater part of the year, may be induced to give the experiment a fair trial.

4. Abstract of Additional Memoir on the Parallel Roads By D. Milne Home, LL.D.

of Lochaber.

Mr Milne Home stated that he had been induced to resume his researches in the Lochaber district, in consequence of a lecture, in June last, by Dr Tyndall of London, at the Royal Institution, Albemarle Street, and subsequently printed and published. In this lecture, Dr Tyndall controverted the detrital theory of lake barriers, and advocated the glacier theory;-supporting his views by a reference to observations which Dr Tyndall said he had recently made on a special visit to the Parallel Roads.

Mr Milne Home said that on account of Dr Tyndall's reputation as a man of science, and his great knowledge of glacier action, he had thought it right to reconsider his own opinions, and go back to Lochaber with Dr Tyndall's lecture in his hand.

During this recent visit, Mr Milne Home said he had obtained farther information bearing on the question, which he hoped might be deemed sufficiently important to be communicated to the Society.

He would first mention the import of this information, and then offer remarks on Dr Tyndall's lecture.

After recapitulating the grounds on which he had in his last Memoir suggested that the barriers of the old lakes had been composed of the natural detritus of the district, he mentioned the following facts, in corroboration of these grounds

1. He specified several additional cases of lakes, in Stratherrick (a district not far from Lochaber), now kept in by detrital blockage, and which, owing to a partial lowering of this blockage, had subsided from higher levels, these higher levels being indicated by horizonal lines of cliff.

VOL. IX.

2 R

2. He specified the occurrence of thick beds of detritus, consisting of clay, gravel, and sand, along the hills between Stratherrick and Strathspey, at levels of from 2000 to 2500 ft. above the sea, -heights far greater than the sites of the barriers of the old Lochaber lakes.

3. He next indicated on a map the probable sites of the barriers by which the lakes of Glen Roy and Glen Spean had been respectively confined, till the barriers were broken down, and gave reasons for fixing on these sites.

4. While in his last Memoir he had attributed the breaking down of the barriers to the agency of the rivers discharging from the lakes, and also of streams flowing upon the barriers from the adjoining mountains, he now added another probable cause of injury, to the barriers at the mouths of Glen Spean and Glen Glusy in the agency of the sea, there being probable evidence that when these barriers existed, the sea was standing at a level of about 500 or 600 feet higher than at present; and that the blockages of these two glens were so near the sea, when at that height, as to form sea-cliffs.

He explained that this evidence consisted, first, in the occurrence of sea shells, said to have been found at two places outside the Glen Spean barrier, in beds of clay from 200 to 500 feet above the sea, by persons whose trusworthiness he had ascertained; and, second, in the existence of a series of terraces, in the district about Spean Bridge, from 400 to 500 feet above the sea, which he considered to be marine.

5. Mr Milne Home then adverted to the grounds on which the glacier theory rested, and stated that by another visit made last autumn to Corry N'Eoin and Glen Treig, the only two valleys, where a glacier is supposed to have existed, when Glens Spean and Roy were occupied by lakes, he had discovered that these valleys, instead of being then filled with ice, must have been occupied by water, viz., the water of the Glen Spean Lake; inasmuch as there are beach lines round the whole of Loc Treig, and also at the mouth of Corry N'Eoin.

6. Referring to what had been called the Moraines of the alleged Treig glacier-viz., at Murlaggan, and in the district between the Rough Burn and Fersit,-he said that at Murlaggan

the knolls referred to were composed entirely of beds of sand and fine gravel,—manifestly deposited by water, and not brought by a glacier.

The supposed Moraines, between Rough Burn and Fersit, consist of enormous embankments of gravel, apparently sub-marine, and formed when the sea stood some thousands of feet above its present level. The boulders strewed over this district, and heaped on these embankments, had probably been brought by floating ice, in a current from the westward, flowing up the valley of Spean towards Strathspey.

7. In regard to Dr Tyndall's lecture, Mr Milne Home pointed out, that when impugning the soundness of the detrital barrier theory, Dr Tyndall had given a representation of that theory only from Sir Thomas D. Lauder's Memoirs, written fifty-nine years ago, and not from the more recently published Memoirs, which stated facts and arguments not to be found in Sir Thomas Lauder's paper. Moreover, Dr Tyndall had unfortunately misapprehended Sir Thomas Lauder's views regarding the formation of the barriers, ascribing to him views on this subject disparaging to the detrital theory, which neither Sir Thomas nor any other author entertained.

The only specific objection to the detrital theory suggested by Dr Tyndall, was the alleged absence of any traces of barriers. Mr Milne Home stated that he thought there were traces; and that, even if there were none, the want of them would be no valid objection to the theory.

With regard to the grounds on which Dr Tyndall maintained the glacier theory, Mr Milne Home stated that Dr Tyndall ought to have shown, how there could be such a difference in the climatic condition of the two sides of Spean Valley, that while the glens on the south side were filled with ice, the glens on the north side were filled with water;-this, however, Dr Tyndall had failed to do.

Mr Milne Home farther stated that though it was undoubtedly true that some traces of glacier action were visible in various parts of Lochaber, these, as it seemed to him, belonged to a period in the world's history long antecedent to the lakes of Glen Roy and Glen Spean. When these lakes existed, the water of the Spean lake penetrated into the two glens (Corry N'Eoin and Treig), supposed to have been occupied by glaciers. It was therefore a physical

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