« PreviousContinue »
twelve times as many oscillations in the secondary as in the primary. In the third and by no means the least important case, the question how close the resonance is does not affect the accuracy of the results. By photographing the sparks in the secondary the period of oscillation is determined, not of a circuit that is altered until by trial it is found to have as nearly as possible the same period of vibration as the circuit on which the length of the wave is measured, but that of the circuit along which the wave itself is actually traveling.
The great difficulty to be orercome is the production of secondary oscillations that will produce sparks of sufficient brightness to photograph. It is comparatively an easy task to photograph the primary spark, but in order to photograph the secondary the dimensions of the circuit must be chosen with great care.
With a view to increasing the light of the spark together with the length of the waves, it seemed desirable to lengthen the period of oscillation by enlarging the condensers rather than by increasing the self induction of the primary circuit. A castor oil condenser was therefore designed and constructed on the following plan. Eight plates (25cm x 20cm) were cut ont of sheet zinc, and were held in vertical planes side by side 2em a part by a suitable hard rubber fraine. The plates were entirely immersed in castor oil contained in a glass jar. They were connected together in the manner shown in fig. 2.
The plates marked a, c and e were fastened to the conductor A B and formed one armature of the condenser. Those marked d, f and h were joined to C D and formed the other armature. The two ends of the secondary circuit E, G, J, H, F, were fastened to the plate b and g. The plane of the secondary circuit was 50cm, and that of the primary Zem above the upper edge of the condenser plate. The total length of the secondary circuit from one condenser plate through E, G, J, H, F, to the other plate was 6338cm "The circuit consisted of copper wire (diameter 215cm) supported at each end by suitable wooden frames, and also once in the middle by hard rubber hooks fastened by long pieces of twine to a wooden crossbar above. The distances from F to E and from K to L were 30cm and a spark yap with pointed tin terminals was inserted at J. The primary circuit consisted of copper wire (diameter •34cm). The distances between the two parts A B and C D was 45cm. The portion B D contained a spark gap with platinum-faced spherical terminals, and was made so as to slide back and forth, to and from the condenser. The motion of this movable piece varied the self induction and therefore the period of oscillation of the primary circuit. By this means the circuits were brought into resonance. With
certain arrangements of the condensers, the resonance was very sharp, and the position of the movable portion could be determined to within .25cm. In the arrangement which was finally adopted the resonance was not so sharp. Even in this case the distance of the sliding part from the plate a could not have been in error by more than 2cm. The length 65cm was finally chosen for its value.
At first it was found impossible to produce anything but a complex vibration in the secondary circuit when the spark gap was open. Some slight evidence of resonance was obtained, but nothing of a decided character. When, however, the spark gap was closed, very good resonance ensued and a wave, the length of which could be measured to within •4 per cent, was excited. Some photographs were taken of the spark in the secondary circuit, and they showed immediately the character of the complex wave formation. The secondary circuit could and did oscillate in three different ways, and the ratios of the periods were those of the notes in an open organ pipe, namely 1:2:3. Usually the lowest or fundamental oscillation together with one of the overtones was present; but several sparks were noticed that furnished unmistakable evidence of the simultaneous existence of all three. We have observed in a circuit 10000cm long the same peculiarities of oscillation, excited by a primary circuit that, judging from its dimensions, could not have been in resonance with the secondary. It was evident that the oscillation having nodes at the points marked (fig. 2) E and F is that whose period is of the fundamental.
FHL A number of measurements of this period have been made, and from these several values the velocity of the waves has been calculated. The results appear in the following table. As an average of five measurements of the wave length, none of which differed from the mean by more than 20cm, the value 5888em was chosen. The distance from the mirror to the photographic plate in each case except the last was 300:1 cm. Each of the first five values in the second column of the table is an average of 30 measurements of distances ranging in the neighborhood of 1cm. The speed of the mirror
was determined to one part in five hundred by means of an
2:819 x 1010
2.810 X 1010
2.835 x 1010
2:808 x 1010
2:808 X 100
Average. 2.816 x 100
2:988 X 1010 The last line in the table contains the results of measurements on photographs of the primary spark instead of the secondary. In this case the distance from the mirror to the photographic plate was 311.5cm. In spite of the fact that the last value of the velocity is much nearer that of the velocity of light, and of the ratio of the two systems of electrical units than the average of the first five, we do not think it can be relied upon, for two reasons. First because of the possible error introduced by the fact that the two circuits had not exactly the same period of oscillation ; and second because the distances measured on the photographic plate were only about ·05cm, instead of 1.00cm
The generally accepted value for the velocity of light is 2:998 X 1010 centimeters. At present it does not seem to us likely, judging from the table as it stands, and from a consideration of the possible errors in the various measurements, that the total error in our determination can be as great as the difference between the average just given and 2:998 x 10.
Whether this discrepancy is due to the fact that the circuit may not have been long enough in comparison with the length of the waves to allow of their full development, or not, we do not undertake to say. If the bends in the circuit at M and M' have a retarding effect upon the waves, this fact can be very easily discovered and allowed for. As yet we have not had time to investigate the question. We therefore publish the results above tabulated as a preliminary record, hoping to refine upon the measurements in several important particulars, and to extend the investigation to circuits of different sizes and shapes, one of which will probably be a long circuit of some 300 meters running out of doors, and at a considerable distance above the ground.
In the final paper, too, we hope to publish a great many details of the method, together with some interesting phenomena that have appeared in the photographs, of the primary and secondary sparks.
If it appears, as theory seems to indicate, that electric waves travel in air with the velocity of light, it may be that the latter can be determined more accurately by an electrical and photographic method than by the eye methods which have hitherto been used.
Jefferson Physical Laboratory.
ART. XXVI.—Epochs and Stages of the Glacial Period; by
WARREN UPHAM. RENEWED studies of the origin and order in age of our Minnesota drift deposits have led me to the results presented in the following table, which I think will contribute toward a reconciliation and harmony of the lately opposing doctrines (1) of unity and (2) of duality or greater complexity of the Ice age. Unity or continuity of our Pleistocene glaciation, with fluctuations of the ice margin, much greater in the interior of the continent than eastward, appears to me the most acceptable view and statement, when the whole period and the whole drift-bearing area are considered. The evidences of a recession of the ice sheet in Minnesota about two hundred miles backward from the nearest portions of its former boundary, followed by growth again nearly to its previous limits, are to be found in The Geology of Minnesota, final report, volumes I (1884) and II (1888), by index references for “Interglacial formations, drainage and water-courses,” etc.
The two stages of growth of the ice-sheet may have been dne, aside from their principal dependence on the high elevation of the land, to the last two passages in the precession of the equinoxes, with accompanying nutation, bringing the winters of the northern hemisphere in aphelion about 30,000 years ago and again about 10,000 years ago. The intermediate time of the earth’s northern winters in perihelion would be the stage of great retreat of the ice margin in the upper Mississippi region ; but eastward, from Ohio to the Atlantic coast, there appears to have been little glacial oscillation.* This explanation accords with Prof. N. H. Winchell's computations from the rate of recession of the falls of St. Anthony for the Postglacial or Recent period,+ and with his estimate of the duration
* J. D. Dana, this Journal, III, vol. xlvi, pp. 327-330, Nov., 1893.
+ Geol. and Nat. Hist. Survey of Minnesota, Fifth Ann. Rep. for 1876, pp. 175– 169; Final Report, vol. ii, 1888, pp. 313–341, with fifteen plates (views showing recent chianges of the falls of St. Anthony, and maps). Quart. Jour. Geol. Soc., London, vol. xxxiv, 1878, pp. 886-901.
of the interglacial stage from the now buried channel which appears to have been then eroded by the Mississippi river a few miles west of the present gorge below these falls. *
Adopting the helpful new nomenclature proposed by Chamberlin,t we may provisionally formulate the minor time divisions of the Glacial and Champlain epochs as follows. The order is stratigraphic, so that for the advancing sequence in time it should be read upward. Champlain Epoch.—(Land depression ; disappearance of
the ice-sheet; partial reëlevation of the land.) WISCONSIN STAGE.—(Progressing reëlevation.) Moderate
reëlevation of the land, advancing as a permanent wave from south to north and northeast; continued retreat of the ice along most of its extent, but its maximum advance in southern New England, with fluctuations and the formation of prominent moraines; great glacial lakes on the northern borders of the United States; slight glacial oscillations, with temperate climate nearly as now, at Toronto and Scarboro', Ont. ; the sea finally admitted to the St. Lawrence, Champlain, and Ottawa valleys; uplift to the present height completed soon after the departure of the ice.
(The great Baltic glacier, and European marginal moraines.) CHAMPLAIN SUBSIDENCE.—Depression of the ice-covered
area from its high Glacial elevation ; retreat of the ice
from its former Iowan limits; abundant deposition of loess. Glacial Epoch.-(Ice accumulation, due to the culmination
of the Lafayette epeirogenic uplift.) IOWAN STAGE. – Renewed ice accumulation covering the
forest beds and extending south nearly to its early bound
ary. (Third European glacial stage.) INTERGLACIAL STAGE.—Extensive glacial recession in the
upper part of the Mississippi basin; cool temperate climate and coniferous forests up to the waning ice-border; much
erosion of the early drift. KANSAN STAGE.-Maximum extent of the ice-sheet in the
interior of North America, and also eastward in northern
New Jersey. (Maximum glaciation in Europe.) UNDETERMINED STAGES of fluctuation in the general growth
of the ice-sheet.-Including an early glacial recession and reädvance shown by layers of interglacial lignite on branches of the Moose and Albany rivers, southwest of
James bay. (First glacial stage in the Alps.) * Am. Geologist, vol. x, pp. 69-80, with three plates (sections and map), Aug., 1892.
+ In two chapters (pp. 724-775, with maps forming plates xiv and xv) of J. Geikie's "The Great Ice Age," third edition, 1894, l’rof. T. C. Chamberlin proposes a chronologic classification of the North American drift under three formations, named in the order of their age, begivning with the earliest, the Kansan, East Iowan, and East Wisconsin formations.