March 8, 1877] 



ATATTJRF 



397 



LETTERS TO THE EDITOR 



\T}ie Editor docs not hold Jdmself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond iinth the ivriters of, rejected manuscripts. 

 No notice is taken of anonymous communications. \ 



Nebulous Star in the Pleiades 



A SHORT paragraph in a recent number of Nature (vol. xv. , 

 p. 244) on the nebulous star in the Pleiades appears to call for 

 a few remarks. With reference to the supposed difficulty of 

 seeing with very large instruments a faint nebulosity in close 

 proximity to a bright star, I may say, that the words of my 

 assistant, quoted in the paragraph referred to, viz., "The 

 Merope nebula is luver perceived with Lord Rosse's telescopes," 

 are perhaps a little too strong. 



The entries relative to this object are five in number. In 

 February, 1871, "Examined under very favourable circum- 

 stances ; no nebulosity seen." August, 1872, " Examined Merope 

 for Tempel's nebula; ; not a ti'ace of nebulosity visible." (Both 

 the above with the 3-foot reflector.) October, 1872, "Tempel's 

 variable nebula not found ; sky clear." September, 1873, 

 "Nothing seen; much false light in field." December, 1S75, 

 " Examined Merope in consequence of M. Tempel's letter (Ast. 

 Nach., No. 2,045) ; "o nebulosity seen ; only same little false 

 light as around the other bright stars; sky very misty." 



It may be expected a priori that imperfections of a certain 

 class, such as dust and other opaque substances, will interfere 

 more with the action of a speculum than of an ol)ject-glass in 

 searching for faint nebulosity near a bright star, inasmuch as in 

 the former they will throw back and disperse over the field light 

 which would otherwise ha\-e contributed to form the image of 

 the star ; whereas in the latter they will cause a general darken- 

 ing by intercepting a certain percentage of light from stars and 

 sky alike. It may therefore still be possible that under peculiarly 

 favourable atmospheric conditions, and with a speculum just re- 

 polishcd, we may still be able to detect the nebulosity, but it 

 appears far more probable that we must lock for an explanation 

 of the difficulty of seeing the nebulosity to the comparative 

 smallness of field of so large an instrument, which in general 

 prevents the simultaneous comparison of the star under observa- 

 tion with neighbouring ones, and of a nebulous sky with an 

 adjacent part free from nebulosity, so well as with a smaller 

 telescope, and to the greater brilliancy of the image of the star 

 while the nebulosity about it is only as bright as in the smaller 

 telescope. From D'Arrest's remarks, quoted in Nature, it 

 appears that such objects are seen with much difficulty with a 

 large refractor also. I have myself noticed, particularly in work- 

 ing with the six-foot reflector on the great nebula in (Drion, that 

 the fainter parts of the nebulosity, whether or not in the vicinity 

 of bright stars, could best be seen with a finding eye-piece of 26' 

 field, of too low magnifying power to utilise more than two- 

 thirds of the diameter of the speculum, and with my eighteen- 

 inch Newtonian, the very faint nebulosity on the preceding side 

 of the nebula could be much better traced.' 



The absence of symmetry of the nebulosity round the star, as of 

 that round t Orionis, should, however, enable real nebulosity to 

 be more easily distinguished from false light than in other cases. 

 The more southern position of M. Tempel's observatory probably 

 gave him some slight advantage. 



It appears to be in the detection of minute stars and the exa- 

 mination of small details, where they exist, rather than in the 

 search for faint ditfused nebulosity, or nebulosity round stars that a 

 large aperture gives so great an advantage. RossE 



"The Movement of the Soil-cap" 



Under the above heading Sir C. W. Thomson gives an inter- 

 esting account of the "stone-rivers" of the Falkland Islands in 

 a recent number of Nature (vol. xv. p. 359), and attributes 

 their origin to a general movement of the " soil-cap." Nothing 

 can be clearer than his explanation of the mode in which the 

 quartzites weather and break up on the hill-slopes, and one can 

 quite understand how the resultant debris is gradually brought 

 down into the valleys by the agents of change he refers to. But 

 it is hard to see how these agents, after having got the dibris 



' P. S. — Although the faint diffused nebulosity preceding tlie nebula in Orion 

 can in general scarcely be delected by any gradations of light within the 

 limits of the field, the general luminosity of the field increasing up to the 

 pebula is strikingly apparent in the six foot. 



down into the valleys, can subsequently spread it out intu wide 

 sheets, reaching " from a few yards to a mile or so in width," 

 and resembling at a distance glaciers that seem as if descending 

 from the adjacent ridges. The stones, as Mr. Darwin tells us, 

 " are not thrown together into irregular piles, but are spread out 

 into level sheets or great streams." Sir C. W. Thomson is 

 apparently of opinion that these great streams of stones move en 

 masse down the valleys, as " earth-glaciers," and he refers to the 

 occurrence in Scotland of certain phenotnena which seem to him 

 to indicate similar movements of the "soil-cap." Geologists 

 who have worked much in hilly countries, will readily recognise 

 the truth of his descriptions — indeed the appearances to which 

 he calls attention are quite common in such districts as the 

 Northern Highlands and Southern Uplands of Scotland. The 

 soil and rock-rubbish which are found resting upon our hill- 

 slopes, and the bending-over of the truncated ends of the 

 underlying vertical cr highly-inclined strata are of course the 

 results of atmospheric action. Rain or thawing snow filters 

 into joints and crevices, and insinuates itself between bedding- 

 planes, and frost tends to force these apart — the loosened 

 rock moving in the line of least resistance, that is, douutt hill. 

 At the same time both solid rock and detached fragments 

 " weather," and thus grit and soil gradually form, while in like 

 manner this gradually-forming "soil-cap " being itself acted upon 

 by frost, is forced in the same way to move down the slope, a 

 movement which is of course aided by a vis a tergo, the weight 

 of the descending mass. Partly in this way, and partly by the 

 direct action of rain, which not only washes the particles down, 

 carrying away surface after surface, but sometimes soaks the 

 loose " soil-cap " to such a degree as to cause the entire accumu- 

 lation to " flow," whole hill-sides become swathed in mantles of 

 soil and debris. But it is difficult to believe that an experienced 

 observer would be puzzled to discriminate between such rubbish- 

 heaps and true glacial moraines. Arrived at the foot of the 

 slope, the rock-rubbish accumulates there, unless there be some 

 stream at hand to denude it, and to sweep its materials, in the 

 form of gravel, sand, and mud, down the valley. There are 

 many good ground*, however, for believing that much of that 

 "suiface-wash " of soil and rock-rubbish which cloaks our hill- 

 slopes to a depth sometimes of many feet, dates back to a time 

 when our climate was considerably colder than it is at present, 

 and that, while it was accumulating, local glaciers occupied 

 many of our mountain-valleys. Putting aside "screes" and 

 dcbris-i\oi^f& generally, I must say I have never seen any indica- 

 tion of that movement en masse of the soil-cap upon which Sir 

 Wyville insists ; and I hardly think many geologists will agree 

 with him that it is " almost self-evident that wherever there is a 

 slope, be it ever so gentle, the soil-cap must be in motion, be 

 the motion ever so slow ; and that it is dragging over the surface 

 of the rock beneath the blocks and boulders which may be em- 

 bedded in it," &c. Soil, as we all know, is always travelling 

 from higher to lower levels, but this movement consists for the 

 most part in the mere sweeping downwards of its component 

 particles by rain and surface-drainage. It is true that the expan- 

 sive power of frost, and the action of vegetation as described by 

 Sir Wyville, may force a certain proportion of a soil-cap en masse 

 down a gentle slope, but these influences will affect only an in- 

 considerable stratum ; and, besides, the movement thus caused 

 will be so trifling that the mere surface-action of rain would 

 suffice to carry away the whole soil, particle by particle, 

 long before the power of frost could have moved it 

 bodily more than an inch or two. In reading the ac- 

 counts of the wonderful " streams of stones " in the Falk- 

 land Island?, one is strongly reminded of the great moving 

 masses of debris in certain valleys of the Rocky Mountains, as 

 described by Dr. Ilayden, and to surmise that the stone-rivers of 

 the Falkland Islands may possibly be of the same nature. Dr. 

 Hayden tells us that entire valleys are " covered thickly with 

 earth, filled with more or less worn rocks of every size, from that 

 of a pea to several feet in diameter. The snow melting upon 

 the crests of the mountains, saturates these superficial earths with 

 water, and they slowly move down the gulch much like a 

 glacier. This is another process of grinding the underlying 

 rocks, smoothing, and grooving them." But he apparently. finds 

 no difficulty in distinguishing between such " earth-glaciers," and 

 the moraines left by those gigantic ice-rivers, which, according to 

 him, flowed down the valleys of the Rocky Mountains during 

 the glacial period. vSuppose now that owing to some change of 

 climate these earth-glaciers were no longer to be saturated with 

 water to such an extent as to cause them to flow en masse, it is 

 evident that the loose soil qf which they are partly composed 



