others, with the sole exception already mentioned, instead of those plants coming in gradually, as they would be expected to do if the formation represented an age at which their development was inchoate, and instead of presenting rudimentary, transition, and archaic types of that subclass, as such early deposits would naturally do, we find them to be the prevailing, sometimes, as in the Dakota Group, almost the only form of plant life, and we also find them fully developed, and even when most unlike our modern vegetation, still exhibiting all the characters of highly organized plants of their rank. In the Potomac formation, on the contrary, we find the Dicotyledons behaving precisely as they ought to behave in a formation that represents an age close down to that at which this form of life first made its appearance in the geologic history of the globe. We find them to constitute the great rarities of the flora, absent from many of the most productive beds, scarce at all places in comparison with the lower types of vegetation, strange and peculiar in character, so vague and ill-defined as in some cases to cause doubts as to whether they really belong to this group of plants, possessing features that recall the ferns, Cycads, Conifers, and even the Monocotyledons, and containing comprehensive types prophetic of many of the now fully developed families of Dicotyledons. They therefore form just such a homogeneous and undifferentiated group of plants, combining in a scarcely distinguishable way all the elements of the later dicotyledonous flora, as we should expect to find existing during the early history of this type of vegetation. They are therefore not to be regarded as anomalous but as normal, and the anomaly, if any there be, exists in Cenomanian floras, where this type occurs in such a predominant and highly developed form. In view of these facts I cannot accept the conclusion that the dicotyledonous element of the Potomac flora argues a more recent age than that denoted by the other types. On the contrary, the immense difference between this and the Cenomanian floras clearly indicates that a vast period must have been required to produce so great a development. On numerous occasions, dating as far back as 1878,* I have expressed the opinion that the Dicotyledons could not have had their origin later than the middle Jura, and it will not sur * American Naturalist, vol. xii, June, 1878, p. 378; November, 1878, p. 734. In a lecture delivered February 24, 1883, at the National Museum on Plant Life of the Globe, past and present (see Science, vol. i, May 4, 1883, p. 358) American Journal of Science, third series, vol. xxvii, April, 1884, p. 302. A. S., vol xxxiii. Philadelphia Meeting, September, 1884, p. 497. Gazette, vol. ix, Indianapolis, October and November, 1884, p. 174. nual Report U. S. Geological Survey, 1883-84, Washington, 1885, diagram, pl. lviii, facing p. 452. Proc. A. A. prise me if the final verdict of science shall place the Potomac formation, at least the lower member in which the plants occur, within that geologic system. While the remaining types point strongly in this direction, I do not regard the Dicotyledons as at all negativing, but even more strongly suggesting this view. Still it may be admitted that according to the ordinary modes of arguing from similar statistics, the sum of all the facts here presented would make the Potomac, considered from the point of view of the flora alone, homotaxially equivalent to the Wealden of England and North Germany, now usually included in the Cretaceous system. If the vertebrate remains are Jurassic and the flora Cretaceous we only have here another confirmation of a law exemplified in so many other American deposits, that, taking European faunas and their correlated floras as the standard of comparison, the plant life of this country is in advance of the animal life. This law has been chiefly observed in our Laramie and Tertiary deposits, but is now known to apply even to Carboniferous and Devonian floras. It is therefore to be expected that we shall find it to prevail during the Mesozoic era. If, therefore, it be finally settled that the fauna of the Potomac series is homotaxially Jurassic, and we take our starting point from Old World geology, there will be no more objection to regarding the Potomac flora as Jurassic than there is now in contemplating the Laramie flora as Cretaceous. In fact, so far as the character of the flora is concerned there is much less difficulty in the case of the Potomac than in that of the Laramie, since I have shown the Potomac flora, viewed in all its bearings, cannot be said positively to negative the reference of the formation to the Jurassic upon the evidence of the plants alone. I do not, however, desire to be understood as arguing for the Jurassic age of the Potomac formation. The most that it is intended to claim is that, if the stratigraphical relations and the animal remains shall finally require its reference to the Jurassic, the plants do not present any serious obstacle to such reference. ART. XIV. Experiments on the Effect of Magnetic Force on the Equipotential lines of an Electric Current; by E. H. HALL, Instructor in Physics at Harvard College. THE experiments of which the following article will give some results have been made in the Jefferson Physical Laboratory of Harvard College at occasional intervals during a period of more than three years. Some of the results have been an ticipated by other investigators. I desire to thank the trustees of the Bache Fund for a liberal appropriation which has been of great assistance in the prosecution of the work. About four years ago, Mr. Shelford Bidwell published what at first appeared to be an explanation of the so-called Hall effect as being due to a thermo electric current set up between strained and unstrained portions of the same piece of metal, an effect which Thomson had discovered and which was well known. The theory thus advanced did not stand the test of examination, but it appeared from a table which Mr. Bidwell gave that, at least, the direction of the effect which magnetic force exerts upon the equipotential lines of an electric current in any given metal could be inferred from the sign of the effect produced by stress upon the thermo-electric property of the metal. Messrs. Coggeshall and W. A. Stone, of the class of 1886 in Harvard College, working with my coöperation, confirmed Mr. Bidwell's table in the case of copper, iron and zinc, but found exceptions to it in French cold rolled steel and aluminium. No other metals were examined by them. In all cases both effects were tested. A note stating most of these facts was published in Science, March 27, 1885. In this Journal for February, 1885, commenting upon the alleged reversal of the "Hall effect," which Mr. Bidwell had found in a strip of gold having two narrow longitudinal slits nearly meeting, I ventured to predict the results of experiments to be made for testing the effect of magnetic force upon the equipotential lines of an electric current in strips of metal, the forms of which may be called variations upon the type used by Mr. Bidwell. For these experiments I have used untempered "French cold-rolled steel" This is described by the dealer who furnishes it as "celebrated for its toughness and superior quality for striking up in die or presswork." This steel is procured in the form of ribbons about 8cm wide. From such a ribbon transverse strips were cut about 21mm wide and in length equal to the width of the ribbon. They were all about 1mm in thickness. Along the middle of these strips longitudinal slits were cut, in some strips one, and in others two in line separated by a certain space. (For illustrative figures see the article mentioned above.) As all the results obtained with these strips were such as might be expected from the considerations given in the article alluded to, and as these considerations have not, so far as I am aware, become the subject of criticism, it seems unnecessary at present to describe either the apparatus or results in detail. The experiments were made in February, 1885. Long Strip and Short Strip.-At the Philadelphia meeting of the American Association in 1884 I gave reasons for think ing that the transverse effect would prove to be less marked near the end than in the middle of a metal strip. Similar reasons led me to suppose that the effect would, cet. par., be less in a short strip, No. 16, than in a long strip, No. 15, both being of F. C. R. steel. Strip No. 15 was like those already described but without any slit. Strip No. 16 was cut from the middle of a strip like 15 and was of about the same width, but only 1.1cm or 1-2cm long. Fearing that soldering connections to the ends of so short a strip as No. 16 would change the character of the strip throughout, I used no solder with Nos. 15 and 16, but made connection with their ends by means of strips of lead pressed firmly down upon them by means of stout clamps. For No. 16 each of the lead strips was made about 4cm long and as wide as the steel in order to give the main current, coming in from the connecting wires, opportunity to spread and become parallel to the edges of the strip before entering the steel. The part of No. 16 not covered by the lead was about 5mm long. Whether the space not touched by the lead was longer, cannot be ascertained, but the extreme length possible was, as the dimensions already given show, about 11cm. Lead was used for making connections partly because its softness made a good contact probable and partly *because, lead showing very little or no transverse action of the sort under examination, its use would practically limit this transverse action to the steel strip. The transverse connections were made, as with the other steel strips mentioned in this article thus far, by means of two stiff German-silver springs, each touching the steel at one point. The considerations which led to the experiments with 15 and 16 were nearly as follows: The transverse action, whatever may be its explanation, makes the equipotential lines run obliquely instead of straight across the strip in which it occurs. If such a strip were short and were joined at each end to another strip in which the same action does not occur, the direction of the equipotential lines in each strip near the junction should be affected by the proximity of the other strip, so that lines in the inert strip would not run perfectly straight across and lines in the active strip would run less obliquely than they otherwise would. If a strip were short, like No. 16, this modification in the direction of the lines might be apparent throughout its whole length, so that the transverse action in a short strip might appear less powerful than in a long strip." May 23, 1885, Nos. 15 and 16 were tested, No. 16 first, then No. 15, finally No. 16 again. These tests were by no means accurate, but they left no doubt that the apparent transverse *See also, upon this point, Ettingshausen and Nernst in the Beiblätter zu den Annalen der Phys. u. Chem., Band xi, Stück. 5. effect was considerably smaller in the short strip than in the long one, the difference amounting apparently to about 20 per cent. STRIPS. DIFFERING IN CROSS-SECTION. Every experiment which bears upon the relation between the transverse action in a strip and the magnetic condition of the strip is of interest. Strips of any strongly magnetic metal under the conditions of these experiments become the more strongly magnetized the greater the ratio of the thickness to the width, but with the non magnetic, or weakly magnetic, metals, no such variation is observable. It is, then, important to ascertain what relation this ratio of thickness to width has upon the transverse action in magnetic and non-magnetic metals. Crosses of Norway iron, of cobalt, of nickel, of silver and of bismuth have been tested with this object in view Silver. From a half-dollar coin (silver 9 parts, copper 1 part) two crosses were made, the dimensions of which were approximately as follows. The smallness of visible effect in these thick pieces of silver made the comparison tedious. The results obtained were nearly as follows, correction being made for the difference in thickness: The strength of field is here given roughly in absolute c. g. s. units. The observations of the first day were less accurate and therefore are entitled to less weight than those of Feb. 10th and July 23d. It appears safe to conclude that in the case of silver the width of the cross has no considerable influence upon the transverse effect. The absolute magnitude of the R. P.* in this metal appeared to be about 750×10-6 ̧ EXT * R. P.= Mx C' where E represents the total transverse electromotive force in c. g. s. units. T represents the thickness of the cross in cm. In previous papers I have not given the R. P's in absolute units, having neglected, as Professor Boltzmann has surmised (Anzeig. d. Kais Akad. in Wien, 1886, Nr. X), to change a resistance from ohms to absolute c. g. s. units. To turn the R. P's given in my article of February, 1885, into absolute c. g. s. units, it is necessary to multiply them by 109. |