Ilia notion or centrifugal force, iibe othpr exam- ffu of the so-called ' furce of inertia,' ia uBed simply to enable us to treat a bod; whose particles are not all moving uniformly tn straight lines as in statical «i}Uilibriun). Thus, by imagining a forcp foiloiring a certain law of intensity actinp outwardly from the earth's aiis, in co-esisience with the force of grarity, we may regard the earth as a stationary boity, subject til these forces. It is the [esullant of tlitse forces which we conimoiily regard aa the force of gravity; and, the earth having assumed the form of eijnillb- rlum, with a surface everywhere nonnal to Ihia re- sultant force, there is no more occasion to consider [be centrifugal force as acting iudependently. I3ut, 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 ihe pole, whether symmetrically situated or not. Of course, it amass 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 ir.sufS- deiit resistance, the ice would ' tlow ' f way in gla- ciers, just as it does from any elevated region of the earth's surface. Wm. Woolsey Joi
��Digestion experiments. An agricultural experiment-«taUon has to contend ft^nst the prejudices of a public which demands speedy work and prepo<>terous generalizations rather than accuracy. When a station, therefore, cloes do work of a scientific character, It is especially de,timus of recognition on the part of science, as such recog- nition not only brings encouragement to the workers. but also has an inSnence upon the public to educate toward bellerenpectallons and wiser demands, What suggests this remark Is an article in Science. April 10, entitled ' Errors in digestion experiments.' from which the reader would iiifer that Professor Armaby's ex- periments upon digeslinn were the only ones of that uharaclcr which have been made in this country. As a matter of reconl, however, I presume Ihe New- York agricultural experiment-station, in Its Bulletin yo. Ixxxv,, Uay IT, 18.'^, Is entitled to the claim of having Urst published the results of a trial upnn the digestibility of a ration tn part composed, in the one case, of corn-fodder, and, in the other, of the same material ensured. In the fortbcoraing report of the station for 18S1, the figures of these trials, as well as of others, will appear In considerable detail.
E. Lawis Stubtkvant.
Ilvnivs, N.V., April 13.
Volcanic dust from south-weateTn Nebraska. There were received at the national tnuseum a few weeks since, from a geiitleman in Nebraska, samples iiC a line white and very sharp dust, supposed bj the sender to he 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 Kepubllcan ttiver. The precise localities given are, Furnas county, two miles south of the Kepubllcan River, in secliona 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 lie has also found simi- lar deposits in Kansas. Colorado, and Wyoming.
An examination of the dust with a microscope shows at once that it U not of Geyser origin, being composed almost wholly of minute fragments oi pumiceous glass, with only very rarely a small particle of hnrnblende. Portions of a coarser deposit, asso- ciated with the dust, contain numeron* rounded frag- ments of felspar, a part of which at least is trlclinlc, as shown by twin striallons, 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
The matter is deemed of sufiiclent importance to mention here, from the fact, that, so far as I am awaro., no deposits of dust of this nature have here- tofore been reported east of the lioi^ky-Mountalo region. Geokue P. Uerrili.
U.S. nsl. mu.min. April 13.
Hastings'a theory of the corona.
Tour reviewer of the ' Report of the eclipse expe- dition to Caroline Island' has, by an unfortunate expression, so entirely misrepresented my theory re-
5arding the sotar corona, that a correction is necessary. [)Sii;ad of supposing that the "coronal plienomena may be fully accounted for by applying the well- kniiwn principles of diffraction," as he asserts, ! 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 certaiuiy 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 thut of the corona. Assuming that the corona is so formed, 1 show that all its characteristics (with the exception of the occ.-isional filamentous structure, where the indication of the theory is doubtful) may be explained naturally and easily, even the polarixatioM and nh!>ence of the Fraun h of er lines. I may venture to describe briefly two observations of Interestgivenindetailin the report, since they have not been noted in thereview, and have been published only in the report. The first Is Profeanor Holden's observation of the so-called ' ahadow-bands ' seen just before and just after totality, and which so strik- ingly suggest a dllTractlou phenometion. 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 admulhs are those of planes tangent to the sun at the polnt-i of second and third contacts, is perhaps not of such Immediate in- terest as the observed reversal of motion, since the latter feature e:icludes the more favored explanatioit which makes them shadows.
The other point is the proof that the '6' group must be regarded as belonging to the same category BB 1474 K, the hydrogen lines II and K; namely, that of bright corona lines. This renders It extremely probable that all the brilliant as well as high chrooio- spherlc lines are also coronal llnea. The fact must be regarded as a strong Indication In favor of the theory advocated. C. S. Habtisos,
Kcw Hsvea. April 13.
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