Feb.iA,, 1889] 



NATURE 



367 



Mass and Inertia. 



Prof. Worthington is perfectly right in saying that in my 

 little book en mechanics I did not carefully and solely use the 

 term inertia in the precise seme I suggested for it in my last 

 letter. The fact that in is really only the coefficient of inertia 

 had not been seized by me when I wrote that book. The idea 

 of calling mass-acceleration inertia simply, was suggested, I 

 believe, by a discussion on Newton's third law of motion in the 

 pages of the Engineer some few years back. It is a suggestion 

 which has gradually commended itself to me, and I am calling 

 the attention of the British Association Committee on Mechanical 

 Units and Nomenclature to it. 



With regard to the other matter referred to by Prof. 

 Worthington, it scarcely strikes one as a satisfactory plan to 

 have one system of units for teaching and another for actual use. 

 Is it not better to get students to tackle dilificulties rather than 

 evade them? Oliver J. Lodge. 



The Crystallization of Lake Ice. 



The percussion figures that Mr. Holland has discovered both 

 are interesting in themselves and seem to be a very handy means 

 of marking off one crystal from another in thin lake ice. Their 

 symmetry about a vertical axis is evidence that the optic axes of 

 the crystals were vertical. The small amount of snow here this 

 winter has afforded unusual opportunities for exan.ining the ice 

 on the Davos lake, and I have found crystals, not indeed equal 

 to those on the Welsh lakes, but still very large. A striking 

 feature in the ice, about a week after it was strong enough 10 

 bear skaters, was the presence of a number of hexagonal disks, 

 of all sizes up to a quarter of an inch diameter, with their planes 

 apparently horizontal. Some were regular hexagons, but gener- 

 ally the i-ides were unequal, though the angles were always 120°. 

 I concluded that within a single crystal all the hexagons would 

 be similarly oriented, and that an interface of two crystals could 

 be distinguished by a sudden twist in the direction of the sides. 

 Judged by this test some of the crystals were at least a foot 

 "broad, and in depth no doubt equal to the thickness of the ice, 

 at that time about a foot. To verify this conclusion Mr. Kidd 

 hacked out a piece with the axe, and we prepared a rough plate 

 six inches long and three thick, which we examined in the 

 polariscope. The rings and cross were easily seen, and the plate 

 proved to be all one crystal with the optic axis vertical in situ. 



These hexagons are not identical with the figures observed by 

 Prof. Tyndall in the path of a sunberm through ice (described 

 in "Forms of Water"), for at the time I saw them the ice was 

 so cold that water froze rapidly in any hole that was made. A 

 friend describes them as looking more like bits of cover-slip glass 

 than anything else. They were formed, I was told, on a day 

 when the warm Fohn wind was blowing, and the ice, no doubt, 

 was at a thawing temperature. But the puzzle is, why they did 

 not vanish when the temperature fell. They reflected light 

 strongly, far more than Tyndall figures, and in some cases showed 

 the colours of thin plates. I noticed that those that gleamed 

 with reflected sunlight often lay considerably to right or left of 

 the vertical plane through the sun. This showed that their 

 planes were (allowing for refraction) inclined sometimes as much 

 as id" or 15° to the horizontal, and inferentially that the optic 

 axes of the crystals were tilted an equal amount from the vertical. 

 I hunted about all over the lake for signs of the columnar 

 structure that I described in my article on "The Plasticity of 

 Ice," but only succeeded in detecting it in one place close to the 

 shore. We cut out a piece there and verified the existence of 

 the columns with the polariscope. There were no hexagons in 

 that part of the ice. 



Of the St. Moritz lake last winter I can only give a very im- 

 perfect account. For at the time I began observations the great 

 depth of snow had sunk the ice, and water had oozed into the 

 snow and there frozen, so that the clear ice was covered with 

 some eighteen inches of hard snow ice. The only part easy of 

 access was where a supply of ice was being cut for the hotels. 

 The process adopted was, after clearing the ice from a certain 

 space, to leave that for a week or two, till the new ice had 

 reached the thickness of a foot, and then cut it again. Both 

 the new ice and the old ice in its neighbourhood was columnar. 

 In one place, however, at <;ome distance from the shore, where we 

 got out a lump of clear ice, we found crystals with the optic 

 axis vertical, and one, at least, three or four inches across. 



Davos Platz, January 29. James C. McCo.nxel. 



Falls of Rock at Niagara. 



The following passages, which will interest geologists, I copy 

 from the Montreal Daily Star of the dates given : — 



"Niagara Falls, Ontario, January 7. — Last Friday evening, 

 about 9 o'clock, a large mass of rock fell from the precipice 

 of the Horse-shoe or Canadian Falls, and on Saturday night, at 

 10 o'clock, another mass broke away. In both cases the noise 

 made alarmed the residents in the vicinity. In the Table Rock 

 House, a stone building, doors were thrown open, and the 

 occupants jumped out of bed greatly excited by the unusual 

 noise and vibration, resembling severe shocks of an earthquake. 

 The same sensation was experienced at the residence of the gate- 

 keeper on Cedar Island, and also half a mile up the river. The 

 effect of these falls on the contour of the cataract is quite marked, 

 the change being from that of an angle at the vertex to the 

 original horse-shoe shape." 



" Niagara Falls, Ontario, January 15. — Another piece of rock 

 broke away from the crest of the Horse-shoe on Sunday night. 

 Although the jar was comparatively slight, the shock was dis- 

 tinctly felt at the Table Rock House. The cataract now pre- 

 sents the extraordinary shape of a double horse-shoe, the smaller 

 one caused by the recent displacement being in advance and to 

 the right of the great horse- shoe. Visitors familiar with the 

 shape of the Canadian Falls during recent years will be able to 

 appreciate the change at a glance. 



" Thousands of people visited the Falls yesterday and to-day 

 to view the relics of the bridge torn down by the late gale, — this is 

 the upper suspension bridge, close to the Falls, destroyed by the 

 storm of last week, — and also to enjoy the magnificent scenery 

 which Niagara always presents when arrayed in her winter 

 apparel. The contract for a new bridge to replace the one 

 destroyed has already been let, and the work will be completed 

 in ninety days." 



Additional facts are here furnished in favour of the opinion 

 that the recession of the great cataract is going on at a rate much 

 more rapid than some have maintained, and more rapid than 

 was estimated by Sir C. Lyell in 1842. Indeed, the rate given 

 by Mr. Bakewell in his work on geology seems to have been 

 nearer to the truth, 3 feet per annum instead of the I foot 

 assumed by the author of the "Principles of Geology." 



Akron, Ohio. E. W. Clay pole. 



Origin of the Radiolarian Earth of Barbados. 



The Barbados infusorial earth is well known for the 

 beautiful specimens of Polycystina which it contains, but con- 

 cerning the rock itself, its geological position, and probable 

 mode of formation, little has been written. 



Schomburgk, in his history of Barbados, gave a general 

 description of it and indicated some of the localities where it 

 had been found, but he did not separate it geologically from the 

 group which he designated the " Scotland formation." One of 

 us having resided in the island for some years has had oppor- 

 tunities of studying the lie of the deposit, and has found that it 

 always overlies the rest of the Scotland beds, and that it gene- 

 rally, although not invariably, intervenes between them and the 

 raised coral reefs which form the surface of the greater portioh 

 of the island. It has been found below the coral in certain 

 borings recently made by the Barbados W'ater-supply Company, 

 and there can be little doubt that it originally formed a sheet of 

 considerable thickness extending beyond the present limits of 

 the island. 



The rock itself varies much in composition : in some places it 

 is almost purely ;iliceous, consisting mainly of Radiolaria and 

 Diatomacea?, whilst in others it is largely calcareous (one 

 sample having yielded as much as 79 9 per cent, of calcium 

 carbonate), containing in places many Foraminifcra. 1 he more 

 siliceous specimens agree closely with the descriptions given of 

 those deep-sea oozes which contain Radiolaria and are more or 

 less destitute of Foraminifcra. We intend to pursue our investi- 

 gation of the deposit, and to compare it, if possible, with samples 

 of modern Radiolarian ooze, but the facts already known to us 

 render it highly probable that the deposit is part of a raised 

 ocean bed. If this conclusion be confirmed, it will correct the 

 prevalent belieef that oceanic deposits are not to be found 

 amongst the rocks which form continents and continental 

 islands, and will at the same time form a strong and well-nigh 

 invincible argument against the theoiy of the permanence of 

 oceans, a theory which has recently V)een discussed and rejected 

 by one of us. J. B. Harrison. 



A, J. Jukes Browne. 



