the higher level upon the boom G1, the effect of the sector C1 wil be to raise the arm H clear of the boom G,, leaving the latter free to take up its equilibrium position under the influence of such small current as may then be traversing the galvanometer-coil, owing to want of balance of the controlling-bridge; thus determining the greater or less descent of the arm H, after the sector C1 has passed from under the comb K, as already explained. As regards the displacement of the cam C, together with the drum D and slider W, along the axis of the shaft S; it is apparent from the foregoing paragraph, that for every revolution of the spindle a displacement of 4 mm. to the left occurs, while a displacement of 8 mm. to the right occurs only for those revolutions in which the arm H misses the boom G1. Thus it follows that when the hit-or-miss arm H just hits the boom G1 as often as it misses, the cam, drum and slider will only suffer small axial displacements on either side of a mean position, but on the whole will be without progressive axial motion. On the other hand, if the demand for heating current is greater than can be met by a steady average of one hit to one miss (that is to say, by one short-circuiting of the series-coils for two complete revolutions of the sleeve X), the galvanometer-boom G1 will be more often deflected to the "cold" side than to the "hot" side, and the misses will preponderate over the hits, In consequence of this, the sector C2 of the cam will operate more often than an average of once in every two revolutions of the sleeve X, and so the displacement of 8 mm. to the right will occur on the whole more than half as often as the displacement of 4 mm. to the left. The result will be a progressive displacement to the right, causing a wider part of the brass sector D to come under the brush J, so that each short-circuiting is of longer duration, and perhaps also causing the slider W to move from one of the blocks L to another, thus reducing the resistance interposed by the series-coils F in the main circuit. These effects will diminish the demand for such frequent short-circuiting of the series-coils; and finally, when the displacement of cam, drum and slider has been carried to such a point, that on an average the arm H hits the boom G1 as frequently as it misses, there will be no further displacement of the cam, drum and slider, which will now execute small excursions on either side of a mean position as before. Quite similarly, a reduced demand for heating current will cause the hits to become more frequent than the misses and to remain so until a sufficient displacement of the cam, drum, and slider in the negative direction has been effected, when the hits and misses will once more occur with equal frequency on the whole.

Phil. Mag. S. 6. Vol. 7. No. 40. April 1904.

2 F

A special feature in the design of the cam C relates to those cases in which the temperature of the controlling-bridge is from any cause persistently too high or (as in starting "all cold") persistently too low. It is essential that in such circumstances the cam should neither become jambed at the end of its run, thus stopping the whole mechanism, nor lose touch with the comb K, so as to fail in operation when the adjustment of temperature has improved. The form given to the cam, and shown in fig. 2, secures this result. It should be mentioned that the comb K on either side of the toothing is cleared away to the level of the throats between the teeth.

I wish to thank Mr. C. Young for the great help he gave me in the design of the instruments. Dr. C. V. Burton has also given me great assistance in writing this paper.

LII. Notices respecting New Books.

Die Dissoziierung und Umwandlung Chemischer Atome. Von Dr. JOHANNES STARK, Privatdozent an der Universität Göttingen. Braunschweig: F. Vieweg und Sohn. 1903. Pp. viii+57.

THE

HE author of this pamphlet has performed a very useful service in rendering accessible to a wide circle of readers the main outlines and present position of a problem which must be regarded as one of the most fascinating ever presented to the man of science-a problem whose vast significance was first seriously thrust on the attention of the scientific public by the epoch-making discoveries connected with that most mysterious element-radium. After explaining the principle of electrolytic dissociation and its application to the theory of conduction in gases, electrolytes, and metals, the author gives an account of Rutherford and Soddy's investigations on the cause and nature of radioactivity, and the evidence in favour of the view that the last link in the chain of transformations going on in radium consists of helium. A most useful appendix contains, in addition to various supplementary explanations, numerous references to the literature of the subject.

LIII. Proceedings of Learned Societies.

GEOLOGICAL SOCIETY.

[Continued from p. 314.]

February 24th, 1904.-J. E. Marr, Sc.D., F.R.S., President, in the Chair.

HE following communications were read:

THE

:

1. Eocene and Later Formations surrounding the Dardanelles.' By Lieut.-Col. Thomas English, late R.E., F.G.S.

Our present knowledge of the older rocks, upon which the Tertiary beds surrounding the Dardanelles rest, only suffices to indicate the positions of the outcrops of a succession of schists, crystalline limestones, granites, and serpentines, which can be traced from the

Ægean district into the Marmora, where they formed an archipelago in the Eocene Sea.

The Eocene deposits surrounding these old rocks commence with sandstones, conglomerates, and clays, which become calcareous and nummulitic upward, and are about 2000 feet thick in the aggregate. They are succeeded by 3000 feet of lacustrine sandstones, clays, and schists, interstratified with volcanic rocks, and containing coalseams. These beds have yielded Anthracotherium, plant-remains, and Corbicula semistriata at the coal-horizon, which is near the middle of the series. They are widely spread in Southern Thrace, and are cut off to the eastward by the falling-in of the Marmora sea-bed. The author has traced them along the Gallipoli Peninsula to Imbros Island,-Lemnos and Samothrake are partly composed of similar beds; and he considers that all these deposits represent the uppermost Eocene and the Oligocene, and that the coal-seams belong to the latter.

The folding of the Lower Tertiary strata is plainly marked, and prolongs the direction of the Greek 'flysch '-deposits into the Marmora, forming basins in which the Miocene beds accumulated.

There are three main folds, all passing east-north-eastward through the Eocene channel between the old rocks of Thrace and those of the Troad. The central fold developed farther eastward in post-Sarmatic times, rising into a ridge at Dohan Aslan, which dammed the outlet for the Marmora water to the west, and was the proximate cause of the formation of the Bosphorus in the Pontic Period, and of the Dardanelles at the end of the Pliocene. Volcanic eruptions were prolonged from Cretaceous to Miocene times in Thrace, Imbros, Lemnos, and Mitylene. Strati Island is entirely volcanic, and the greater part of Imbros also.

Marine Miocene (Helvetian to Tortonian) deposits appear north of the Gulf of Xeros and in the Marmora, and are probably vestiges of a Lower Miocene sea-connection between the Ponto-Caspian and the Mediterranean.

Sarmatic deposits, first freshwater, then marine, result from the development of a lake, with a narrow opening north-eastward to the Pontic area, which occupied a large portion of the district. The freshwater beds are still nearly horizontal in the Dardanelles, but are much dislocated along the northern shore of the Sea of Marmora, where they contain naphtha and lignite. The overlying marine (Mactra) limestones fringe the freshwater beds as a shore-belt for 30 miles along this coast, and extend through the Dardanelles to the Southern Troad.

Brackish and freshwater Pontic strata occur in numerous detached lake-basins which drained north-eastward. The Bosphorus was probably cut by river-action through the rim of the lowest of these basins, on the recession of the Sarmatic Sea, and the Ægean drainage then passed into the large, closed, brackish lake described by Andrussov as occupying the Black-Sea area from the Pontic to the beginning of the Diluvial Period.

The water-line of this sea-lake finally receded to nearly 200 feet below its present shore-line, when the Sea of Marmora stood about 80 feet higher. Then the water began to rise again during the