July 5, 1883] 



NA TURE 



2 37 



so, this alternate motion ought to have been noticed in the 

 various observations which have been made upon glaciers, and 

 this does not appear to have been the case. But, in reply to 

 this, it may be remarked that most of the observations have only 

 given the net movement of points on the glacier during intervals 

 of a day or more, and therefore would not show the oscillations. 

 Again, such observations have always been at points near the 

 end of a glacier. Now the variations in temperature of a 

 glacier will be very different at different parts, and the motion of 

 the end of the glacier will, to a great extern, show the average result 

 of these different advances and retreats in different parts of the 

 higher regions. This average result will, of course, be a steady pro- 

 gression down the valley, and the oscillatory movement at the end 

 of the glacier may be so much masked by this as not to be readily 

 observable. Lastly, it may be suggested as possible that a certain 

 amount of expansion by heat may have the effect of giving a sit 

 to ice, so that it does not return to its original length when 

 brought back to the same temperature. If this he so, the oscilla- 

 tions would be much less marked, and at the end of the glacier 

 would probably be indistinguishable. 



I may now draw attention to some phenomena of glacier 

 action, which are explained by the heat theory, but which do not 

 seem explicable on the gravitation theory. 



(1.) It is well known that glaciers, when they emerge from a 

 narrow gorge into a comparatively w ; de valley, spread out into 

 a fan shape. The Rhone glacier is a well-known instance. A 

 still better one is a small glacier in Norway, menti >ned by Prof. 

 Sexa, which spreads out to five or six times its previous width. 

 Now the effec of gravity, acting on a mass as a whole, is to 

 carry it in one single direction, that of the steepest slope. The 

 only way in which gravity can produce such a spreading out is 

 by the parts of the glacier shearing over each other in the manner 

 of a viscous solid. Cut the phenomena of ice cliffs, as men- 

 tioned above, show that ice does nut spread from this cause, so 

 that the fact seems impossible to explain by gravitation alone. 

 On the heat theory it is, of course, perfectly easy : the expansion 

 and contraction will take place in all directions where there is 

 freedom to move. 



(2.) Connected with this phenomenon is that of the longi- 

 tudinal crevasses seen near the edges of glaciers, and particularly 

 where they spread out in the manner just described. Now on 

 the gravitation theory, as remarked above, the only possible 

 explanation of a crevasse is that the ice is bending over a convex 

 surface, and that its upper part is thus placed in a state of 

 tension, under which it breaks. Since, on the gravitation theory, 

 every part of a glacier is exposed to a severe pressure from 

 behind, this explanation does not fit very well even for transverse 

 crevasses ; but to longitudinal crevasses it is clearly inapplicable, 

 since the bottom of a valley is seldom or never convex in the 

 direction of its width. On the heat theory the explanation is 

 simple. We may suppose the heat energy communicated per 

 square foot of surface to be about the same, whether near the 

 middle or edge of a glacier. This energy is expended in pro- 

 ducing an expansion throughout the whole thickness of the 

 glacier, as described above. Hence the smaller this thickness, 

 the greater will be the amount of expansion, and the greater 

 therefore the net motion which results. Hence the thinner 

 parts of a glacier will always be tending to tear themselves off 

 from the thicker, and thus longitudinal crevasses will frequently 

 be found. 



(3.) The striae which are so marked a feature of glacier-worn 

 rocks become more easily explained on this theory. I have seen 

 such striae, even in the hard hypersthene of Skye, which were a 

 considerable fraction of an inch in depth. When we consider 

 the enormous force necessary to plough out such a furrow in 

 hard rock, it is almost impossible to believe that it was done by 

 the simple passage over it, once for all, of a stone imbedded in 

 the ice. If, however, the stone descended by a series of oscil- 

 lations, so that it passed many times over the same spot, this 

 difficulty is greatly lessened. 



(4. ) In conclusion I may point out that the advocates of the 

 gravitation theory are bound to explain what becomes of the 

 heat energy which is poured into a glacier. When the sun is 

 shining this radiant energy is always very large, although the 

 temperature of the air may be low. In such cases the glacier 

 does not melt ; it is perfectly clear that it must expand, as any 

 other solid must expand under the action of heat. If so, it 

 seems unreasonable not to hold that the gradual descent by alter- 

 nate expansion and contraction must follow, as it is known to 

 follow in the case of other materials. 



On the subject of the motion of Arctic ice, Dr. Rae, F.R. S., 

 has kindly permitted the publication of the following par- 

 ticulars : — 



"When in Greenland, in the autumn of 1866, I was ice- 

 bound at the head of one of the fiords, and slept a couple of 

 nights at an Eskimo's house. A glacier about half a mile distant 

 was then in full activity, the movement of which might, I 

 believe, have been as visible to the eye as it certainly was 

 audible to the ear. 



" My own idea is that Arctic glaciers must have a downward 

 motion more or less during the whole year, summer and winter. 

 I believe the alternation of heat and cold — or, I should rather 

 say, of temperature — would of itself cane motion, especially 

 near the upper surface. 



" We know that ice 2 or 3 feet or more thick contracts very 

 considerably in a few hours by a sudden fall of 15 or 20 degrees 

 of temperature. I have found cracks in Lake Winnipeg 3 or 4 

 feet wide, formed by this cause during a single night, almost 

 stopping our sledge journey. This gap soon freezes up. Then 

 the weather gets milder, the ice expands, and with the new addi- 

 tional formation is too large for the lake, and is forced up into 

 ridges. This process goes on at every 'cold snap,' 1 alternating 

 with milder weather. Now supposing a glacier for 10 or more 

 feet of its depth contracts by cold, as lake ice is known to do, it 

 will get a series of cracks probably in its longest axis, say from 

 inland seaward ; the first snowdrift will fill up these cracks or 

 some of them, and this filling up will to some extent perform 

 the same office as the freezing of the cracks in the lakes. The 

 longitudinal extent of the glacier will be increased. A snowstorm 

 always brings milder weather, which would expand the glarer, 

 but as this expansion would naturally tend downhill, instead of 

 up, the whole motion would be downwards. But even if the 

 cracks I mention did not take place, the contraction by cold 

 would pull the ice downhill, not up, whilst the expansion by 

 increase of temperature would tend \o push the glacier downhill, 

 so that these opposite actions would produce similar effects in 

 moving the glacier, or such part of it as could be acted upon by 

 external temperature, downwards. 



"I may also add that when a crack, however slight, is formed 

 by contraction, the cold is admitted into the body of the glacier, 

 and increases the contracting power or influence." 



SCIENTIFIC SERIALS 



Annalen der Physik und Chimie, xix., part 4. — Electrical 

 experiments : electric pressure on solids, by G. Quincke. This 

 paper forms a continuation to a series of experiments in electro- 

 statics published by the author in previous numbers of the 

 Annalen, under the title of "Electric Expansion." It is illus- 

 trated with twenty-six cuts, and will be followed by a communi- 

 cation on the resistance of insulating fluids to electric force. — 

 On electric disturbance at contact of gases with bodies in 

 combustion, with four illustrations, by Julius EI-te and Hans 

 Geitel. The authors arrive at the general conclusion that all 

 flames may be regarded as streams of hot gas, which generate 

 negative electricity in burning electrodes introduced from with- 

 out, as well as in small bodies in combustion suspended in tbem. 



On electric vibration, and more especially on the phenomena 



of polarisation produced by vibratory movements, with four 

 illustrations, by A. Overbeck. — On the dependence of gases as 

 heat conductors on the state of the temperature, with three illus- 

 trations, by A. Winkelmann. — On the fundamental equations of 

 E. Ketteler's theory of optics, by W. Voigt. The author shows 

 that, so far from flow'ing from the principles of the doctrine of 

 elasticity, Ketteler's fundamental equations are diametrically 

 opposed to them. 



The Journal di Physique (May, 1883) contains the following 

 original papers :— On the difference in barometrical pressure at 

 two points in the same vertical line, by J. Jami.i.— On the action of 

 heat upon boracite and upon sulphate of potash, by E. Mallard. 



r> n t he penetration of actinic rays into the eye of man and 



that of vertebrates, and on their vision of ultra-violet rays, by 

 Ede Chardonnet. — On a new apparatus for verifying the laws 

 governing the fall of bodies, by M. Paquet.— On an experimen- 

 tal demonstration of the unequal velocity of the transmission of 

 sound in gases and solids, by F. Griveaux. 



1 "Cold snap," an American term meaning a rather sudden increase of 

 cold. 



