April 13, 1900.] 



SCIENCE. 



591 



(soaked up in powdered charcoal) as an explo- 

 sive. These experiments have not fully de- 

 termined the usefulness of this cheap explosive, 

 the principal difficulty being that the mixture 

 changes its composition rapidly as the nitrogen 

 and oxygen evaporate. 



The author suggests that the most promising 

 field for liquid air machines is in their use for 

 separating (partially) the oxygen from the large 

 amount of nitrogen with which it is associated 

 in the atmosphere. 



ATMOSPHERIC ELECTRICITY. 



At last we have a reasonable theory of atmos- 

 pheric electricity based upon facts. Elster and 

 Geitel, and independently J. J. Thomson and 

 C. T. R. Wilson, have applied the known prop- 

 erties of ionized gases to the explanation of 

 atmospheric electricity. The sun's light, es- 

 pecially the ultra violet rays, ionizes the at- 

 mosphere producing equal numbers of positively 

 and negatively charged ions. These ions are 

 ordinarily present in equal numbers in dry air 

 and their charges do not therefore develop any 

 perceptible electric potential. When the air is 

 cooled below its dew point, however, the nega- 

 tive ions mainly serve as nuclei upon which the 

 moisture is condensed in drops which in fall- 

 ing remove the negative ions, leave an excess 

 of positive ions; this excess of positive ions gives 

 rise to very great electric potentials, aud pro- 

 duces the electrical effects which accompany 

 rain. The reasonableness of this theory is that 

 every physical action which enters into it has 

 been followed in the laboratory — the ionization 

 of air by ultra violet light, the condensation of 

 moisture on the negative ions, etc. 



W. S. F. 



CURRENT NOTES ON PHYSIOGRAPHY. 



GLACIAL LAKES IN WESTERN NEW YORK. 



Fairchild has extended his studies of glac- 

 ial lakes to the Finger lake district of western 

 New York, aud presents a comprehensive sketch 

 of nineteen valleys in which such lakes were 

 formed, several of them showing shore lines at 

 successive levels (Bull. Geol. Soc. Amer. , X. , 

 1899, 27-68). The southward overflows of the 

 lakes, leading over passes between the hills of 



the Allegheny plateau to the different head- 

 water streams of the Allegheny and Susque- 

 hanna rivers, are enumerated and figured. The 

 eastward outlets, between the northern slope of 

 the plateau and the retreating front of the ice, 

 previously described by Gilbert and Quereau, 

 are here beautifully illustrated ; they prove to 

 be even stronger topographic features than the 

 channels of similar origin carved in the drift of 

 eastern Michigan, as described by Taylor. 



Eeference should be made in this connection 

 to a thesis on ' Some higher levels in the post- 

 glacial development of the Finger lakes of New 

 York State,' submitted by T. L. Watson, a 

 graduate student in Cornell University in 1897 

 (Rept. Director N. Y. State Museum, 1898, 

 App. B, 65-117). 



THE POMMEEANIAN COAST-LAND. 



Details concerning the course of valleys 

 formed by rivers marginal to the retreating ice 

 of north Germany recently given by Keilhack 

 (Die Stillstandslagen des letzten Indlandeises 

 und die hydrographische Entwickelung des 

 pommerschen Kiistengebietes, Jahrb. k. preuss. 

 geol. Landesanstalt, 1899, 90-152, 14 maps) 

 supplement the general account referred to in 

 Science for January 5, 1900. After the ice 

 sheet had withdrawn from the morainic hills 

 of the Pommeranian lake belt, twelve succes- 

 sive stages of constrained drainage are recog- 

 nized and mapped, interrupted terminal mo- 

 raines having been formed during some of the 

 stages, and special conditions of marginal drain- 

 age having characterized every one of them. 

 The important valleys eroded in the uplands 

 by rivers marginal to the retreating ice are 

 analogous to those above described, in the 

 northern spurs of the Allegheny plateau of west 

 central New York. If any one desires novel 

 evidence of the former existence of land ice- 

 sheets, and of their importance in fashioning, 

 directly, or indirectly, existing geographical 

 features, it may be found in abundance in the 

 two districts here referred to. 



GERMAN PHYSIOGRAPHIC TERMS. 



A CHAPTER on the Earth's Surface, written by 

 Penck for Scobel's ' Geographisches Handbuch 

 zu Andrees Handatlas ' (3d ed., 1899), presents 



