790 



SCIENCE. 



[N. S. Vol. XI. No. 281. 



employed by A. P. Chattock in case of dis- 

 charge from point to plate. The question of 

 the slow decay of the e. m. f. between the car- 

 bons of an arc, after the current ceases to flow 

 (circuit broken), can be approximately answered 

 in terms of the ionic theory. This e. m. f. would 

 die away as the clouds of positive and negative 

 ions near the carbon tips diffuse towards each 

 other. The time required for this would be in 

 the neighborhood of 10V7 second for an arc 1 

 cm. long, if we assume an ionic velocity of 3000 

 cm. per second per electrostatic unit of poten- 

 tial gradient, and the value of the counter 

 e. m. f. at the instant of breaking the circuit 

 (which would also be the real counter e. m. f. of 

 the arc while running) could be easily calculated 

 from Blrs. Ayrton's curve. Thus the curvature 

 of Mrs. Ayrton's curve is the density of charge 

 at each point ; and from this the potential fall 

 from carbon to carbon is easily calculated. 



Practically, Mrs. Ayrton's curve taken in 

 conjunction with the ionic theory of the arc 

 settles the perplexing question of the counter 

 electromotive force of the arc. Consider the 

 freshly dissociated ions along the path of the 

 arc. The positive ions have to be hauled up by 

 the impressed e. m. f. into the cloud of positive 

 ions, and the negative ions have to be hauled 

 down into the cloud of negative ions. The 

 work so spent is reversible, except that energy 

 is being continuously dissipated at the carbons, 

 as the ions in the clouds lose their charges. 

 A small amount of energy is also dissipated be- 

 cause of the viscous drag of the arc vapors upon 

 the ions. W. S. F. 



CURRENT NOTES ON PHYSIOGRAPHY. 



SHORELINE TOPOGRAPHY. 



A SUCCESSFUL attempt has been made by F. 

 P. Gulliver to trace a sequence in the develop- 

 ment of shoreline forms, distinguishing those 

 which are produced in the earlier stages from 

 those which characterize the later stages of 

 what may be called the 'shoreline cycle.' A 

 large number of littoral forms recorded on maps 

 from all parts of the world were thus classified 

 in accordance with the processes of marine 

 erosion as determined by local observation and 

 general study, the results of the work appearing 

 in a thesis entitled : ' Shoreline Topography ' 



(Proe. Amer. Acad., xxxiv, 1899, 177-2.58, 32 

 figures.) The author considers first the shore- 

 lines due to relative change of level of land and 

 sea, and as yet essentially unmodified by sea 

 action : these are the initial forms, on which the 

 agencies of change then proceed to develop a 

 long series of sequential forms, until interrupted 

 by later movement. Systematic description and 

 explanation is thus given to a large number of 

 shore forms, such as cuspate forelands, ofi"-shore 

 bars, bars by which islands come to be tied to 

 an adjacent mainland, bay-bars, spits, deltas, 

 cliffs, and so on. Under each heading, a type 

 example is selected and usually figured ; ad- 

 ditional examples are serviceably indicated by 

 specific references to maps from many coasts. 

 A bibliography of 100 titles is appended. 



SHORE FORMS IN THE BEAS D'OB LAKES. 



Tarr's account of cuspate forelands in the 

 Bras D'Or Lakes of Cape Breton Island {Amer. 

 OeoL, xxii, 1898, 1-12), is followed by a simi- 

 lar paper by Woodman {Amer. OeoL, xxiv, 

 1899, 329-342), describing additional shore 

 features of the same irregular water bodies, 

 where cusps, looped bars, single and double 

 tombolos, and bars across the mouth, middle 

 and head of bays are developed in remarkable 

 variety. Through both these papers there 

 seems to be some misapprehension of the share 

 of work in making cuspate forelands attributed 

 to waves and currents by Gulliver in his essays 

 on the topography of the shore line ( Cuspate 

 forelands. Bull. Oeol. Soc, Amer. vii, 1896, 

 399-422, and Shore-line Topography, as above ). 

 The former authors explain the cusps that they 

 observed solely by what they regard as wave- 

 action. The latter author refers the 'long-shore 

 transportation that is involved in the production 

 of cusps to currents which, in inland and tide- 

 less water bodies, he regarded as of wind origin. 

 In so doing, it does not seem to have been his in- 

 tention in the least to exclude from waves the 

 power of moving shore materials, but to analzye 

 the forces acting on a shore, much as had been 

 done some years before by Gilbert, who wrote : 

 " Usually, and especially when the wind blows, 

 the water adjacent to the shore is stirred by a 

 gentle current flowing parallel to the water 

 margin. This carries along the particles of 



