April 24, 1885.] 



SCIENCE. 



335 



The notion of centrifugal force, like other exam- 

 ples of the so-called ' force of inertia,' is used simply 

 to enable us to treat a body whose particles are not 

 all moving uniformly in straight lines as in statical 

 equilibrium. Thus, by imagining a force following a 

 certain law of intensity acting outwardly from the 

 earth's axis, in co-existence with the force of gravity, 

 we may regard the earth as a stationary body, subject 

 to these forces. It is the resultant of these forces 

 which we commonly regard as the force of gravity; 

 and, the earth having assumed the form of equilib- 

 rium, with a surface everywhere normal to this re- 

 sultant force, there is no more occasion to consider 

 the centrifugal force as acting independently. But, 

 if we choose to do so, then we must regard the ra- 

 dial force of gravity as acting also ; and the centrifu- 

 gal force acting at any point is then balanced by the 

 force which would, if the earth were not in rotation, 

 reduce it to a spherical form. Thus the centrifugal 

 force can create no tension in an ice-cap, and there 

 is nothing in the nature of the forces acting to inter- 

 fere with the existence of a continuous ice-cap round 

 the pole, whether symmetrically situated or not. Of 

 course, if a mass of ice were piled up at the pole 

 above the spheroidal surface of equilibrium, lateral 

 pressure would exist, but only in the same way that 

 it would under like circumstances in any other part 

 of the earth; and, wherever this pressure met insuffi- 

 cient resistance, the ice would ' flow ' away m gla- 

 ciers, just as it does from any elevated region of the 

 earth's surface. Wm. Woolsey Johnson. 



Digestion experiments. 



An agricultural experiment-station has to contend 

 against the prejudices of a public which demands 

 speedy work and preposterous generalizations rather 

 than accuracy. When a station, therefore, does do 

 work of a scientific character, it is especially desirous 

 of recognition on the part of science, as such recog- 

 nition not only brings encouragement to the workers, 

 but also has an influence upon the public to educate 

 toward better expectations and wiser demands. What 

 suggests this remark is an article in Science, April 10, 

 entitled ' Errors in digestion experiments,' from which 

 the reader would infer that Professor Armsby's ex- 

 periments upon digestion were the only ones of that 

 character which have been made in this country. As 

 a matter of record, however, I presume the New- 

 York agricultural experiment-station, in its Bulletin 

 No. lxxxv., May 17, 1884, is entitled to the claim of 

 having first published the results of a trial upon the 

 digestibility of a ration in part composed, in the one 

 case, of corn-fodder, and, in the other, of the same 

 material ensilaged. In the forthcoming report of 

 the station for 1884, the figures of these trials, as well 

 as of others, will appear in considerable detail. 



E. Lewis Sturtevant. 



Geneva, N.Y., April 13. 



Volcanic dust from south-western Nebraska. 



There were received at the national museum a few 

 weeks since, from a gentleman in Nebraska, samples 

 of a fine white and very sharp dust, supposed by the 

 sender to be of geyser origin. The deposits from 

 which the samples were taken are stated to be semi- 

 circular in outline, from four to ten feet in thickness, 

 and of varying grades of fineness, situated on the 

 banks of small streams that flow into the Kepublican 

 River. The precise localities given are, Furnas 

 county, two miles south of the Republican River, in 

 sections 9 and 10, township 3, north range 21 west; 

 and Harlan county, one mile south of the river, sec- 



tions 10 and 11, township 2, north range 20 west; 

 though the writer states that he has also found simi- 

 lar deposits in Kansas, Colorado, and Wyoming. 



An examination of the dust with a microscope 

 shows at once that it is not of Geyser origin, being 

 composed almost wholly of minute fragments of 

 pumiceous glass, with only very rarely a small particle 

 of hornblende. Portions of a coarser deposit, asso- 

 ciated with the dust, contain numerous rounded frag- 

 ments of felspar, a part of which at least is triclinic, 

 as shown by twin striations, and hornblende and mag- 

 netite particles. The deposits are therefore, without 

 doubt, volcanic dust and sand, owing their present 

 arrangement to the assorting agency of water and 

 atmospheric currents ; and their mineral composition 

 would indicate that the corresponding lava was an 

 andesite. 



The matter is deemed of sufficient importance to 

 mention here, from the fact, that, so far as I am 

 aware, no deposits of dust of this nature have here- 

 tofore been reported east of the Rocky-Mountain 

 region. George P. Merrill,. 



U. S. nat. museum, April 13. 



Hastings's theory of the corona. 



Your reviewer of the ' Report of the eclipse expe- 

 dition to Caroline Island ' has, by an unfortunate 

 expression, so entirely misrepresented my theory re- 

 garding the solar corona, that a correction is necessary. 

 Instead of supposing that the " coronal phenomena 

 may be fully accounted for by applying the well- 

 known principles of diffraction," as he asserts, I 

 demonstrated that these principles completely fail to 

 account for any part of them. What I did do was to 

 prove that Fresnel's theory is not applicable to the 

 case where both source of light and screen are at an 

 indefinitely great distance from the observer; for then 

 the implied constancy of phase of the wave-surfaces 

 certainly does not exist. This limitation of the 

 theory of diffraction does not seem to have been noted 

 before; and it necessarily implies a distribution of 

 light about the moon during a total eclipse which 

 may be like that of the corona. Assuming that the 

 corona is so formed, I show that all its characteristics 

 (with the exception of the occasional filamentous 

 structure, where the indication of the theory is 

 doubtful) may be explained naturally and easily, even 

 the polarization and absence of the Fraunhofer lines. 



I may venture to describe briefly two observations 

 of interest given in detail in the report, since they have 

 not been noted in the review, and have been published 

 only in the report. The first is Professor Holden's 

 observation of the so-called ' shadow-bands ' seen 

 just before and just after totality, and which so strik- 

 ingly suggest a diffraction phenomenon. No one 

 before him, so far as I know, has determined with 

 any useful precision their azimuth, nor had it before 

 been recognized that they move in opposite directions 

 at the two epochs. That their azimuths are those of 

 planes tangent to the sun at the points of second and 

 third contacts, is perhaps not of such immediate in- 

 terest as the observed reversal of motion, since the 

 latter feature excludes the more favored explanation 

 which makes them shadows. 



The other point is the proof that the ' b ' group 

 must be regarded as belonging to the same category 

 as 1474 K, the hydrogen lines H and K; namely, that 

 of bright corona lines. This renders it extremely 

 probable that all the brilliant as well as high chromo- 

 spheric lines are also coronal lines. The fact must 

 be regarded as a strong indication in favor of the 

 theory advocated. C. S. Hastings. 



New Haven, April 13. 



