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244 



NATURE 



[July 13, 1893 



transient nature, speaks of it as a great flood. His flood 

 deposits are not, however, the same as those referred to 

 by Prof. Prestwich. 



Sir Robert Ball has pointed out that if the heat 

 received in winter is distributed over thirty-three more 

 days, instead of only over seven more, the result would be 

 glacial conditions in the northern hemisphere, a result 

 which has been somewhat modified by Mr. Hobson, who 

 pointed out that the heat received over the regions 

 within the Arctic circle should be omitted from the 

 calculation. 



But the opponents of the astronomical theory are pre- 

 pared to admit that when we are dealing with operations 

 in which the effects are so obviously cumulative and the 

 reaction of one on the other so important, we may expect 

 in climate, as there are in what we call weather, times of 

 such unstable equilibrium that, for instance, a slight pro- 

 longation of the period of cold, which may be small in 

 itself, may yet cause a local change, the effects of which 

 may eventually become very far-reaching — as, in the 

 case of weather, rain may be produced by the explosion 

 of a small quantity of dynamite in the upper regions of 

 the atmosphere. It is useless to deny the existence of 

 such causes as those on which the advocates of the 

 astronomical theory chiefly rely any more than we 

 should deny the possibility of the detection of tidal 

 action in one of the American lakes, because we are con- 

 vinced that the real cause of the rise of six feet or so of 

 water on one side is due to the gale which we observe 

 blowing across the lake. Nor are we justified in reject- 

 ing the possibility of more or less rapid submergences re- 

 sulting in a rearrangement of surface debris or even in 

 more cataclysmic action of the same kind, as was seen 

 in the earthquake wave that rolled in on Lisbon. 



We do seem to require some simpler theory than that 

 of the extreme glacialists to explain the -phenomena of 

 the Pleistocene Age. 



If the north-eastern portion of America and the north 

 western part of Europe were raised so as to get a snowy 

 mountain range on either side of the Atlantic, sending 

 ice-sheets down to the sea in the intervening depressed 

 trough, and by the convergence of the axes of elevation 

 deflecting the ocean currents and causing glaciers to 

 creep down east and west from the mountain ranges — all 

 the phenomena of the glacial epoch could be explained. 



Reverse the process ; send up Greenland and lower 

 the North American and Scandinavian chains even to 

 below where they now stand, bringing in again the warm 

 currents from the South, and the post-glacial submerg- 

 ence takes its place. Let the Icelandic volcanic system 

 play its part, and let there be earthquakes and jerks and 

 oscillations, all part of the regular course of operations 

 accompanying such movements, and we have the marine 

 drifts all explained. The forms of life which have been 

 driven away from the centres of ice dispersal will follow 

 the receding glaciers back again. Observers will find in 

 their own district evidence of land ice, or of icebergs, or 

 of sea currents, or of glacier water, but in this less 

 cumbrous theory there will be room for all. 



The conflict of views recorded in Sir Henry Howorth's 



exhaustive work prepares one to believe that the matter 



may not be finally settled for some time, and, before public 



opinion comes to rest, we may expect many swings of the 



NO. 1237, VOL. 48] 



pendulum now far on this, now far on that side of the 

 truth. But we welcome this protest against the extrava- 

 gant views of the extreme glacialists and this valuable 

 encyclopaedia of the facts and arguments bearing on 

 glacial phenomena which must be in the hands of every 

 student of the subject. Our author is well known for 

 his scientific treatment of literary subjects and for the 

 literary skill with which he presents his scientific facts. 

 Though he is an uncompromising advocate of what 

 commends itself to him as the right view, he has indulged 

 in no criticism which can be regarded as discourteous to 

 the living or unfair to the dead. 



T. McKenny Hughes. 



DYNAMO-ELECTRIC MACHINERY. 

 Original Papers on Dynamo Machinery and Allied 



Subjects. By J. Hopkinson, M.A., D.Sc, F.R.S. 



(New York : The W. J. Johnston Company, Limited. 



London: Whittaker and Co., 1892.) 

 The Dynamo. By C. C. Hawkins, M. A., A.I.E.E., and F. 



Wallis, A.I.E.E. (London : Whittaker and Co., 1893.) 



THERE is hardly any greater authority on the subject 

 of dynamo-electric machinery than Dr. Hopkinson. 

 It was he who, turning his attention to the Edison machine, 

 first showed how the iron in the magnets should be dis- 

 tributed, how the magnetising coils should be wound, 

 and the machine built up so as to ensure its possessing 

 the highest possible efficiency in every sense of the word. 

 This he did not attempt to do by mere theoretical 

 speculation, though himself a great theorist, but by in- 

 stituting a very complete and exhaustive set of 

 experiments on dynamo machines under practical con- 

 ditions, and graphically representing their results. No 

 device in the whole history of the evolution of the 

 dynamo has been of more general service than his plan 

 of exhibiting the results of experiments in the well- 

 named characteristic curve of the machine. This did for 

 the dynamo what the indicator diagram had long been 

 doing for the steam engine, though not, of course, in the 

 same way. 



With the most admirable simplicity this curve of electro- 

 motive forces as ordinates, and currents as absciss;e, gave 

 just the information required regarding the action of the 

 machine. Thus, when the ordinates represented the 

 potential diff"erences between the terminals, the inclination 

 to the axis of abscissje of the line joining the origin to 

 any point gave the working resistance in the external 

 circuit, corresponding to the current and potential differ- 

 ence defining the point to which the line was drawn, or^ 

 this resistance being known, gave the current and potential 

 difference which the machine might be expected to de- 

 velop with this as the working part of the circuit. 



Then again. Dr. Hopkinson showed how the charac- 

 teristic curve could be used to give the conditions under 

 which an arc lamp can be made to work. It is well 

 known that if the generating machine working on an arc 

 lamp be run so as to give an electromotive force below a 

 certain limiting value, the machine cannot be made to | 

 " keep " an arc. An explanation had been previously I 

 given by Dr. Siemens ; but Dr. Hopkinson showed that 

 all that was necessary was to lay down in the character- 

 istic curve of the dynamo as already explained the line 



