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yielded only gastrulæ. Finally, hybrids, of a presumably intermediate character, were obtained from the cross Echinus microtuberculatus ? and Echinus acutus. With the ova of Sphærechinus and Echinocardium, and the sperm of Echinus acutus, only gastrula were obtained.

On performing cross-fertilisations with the colour varieties of Sphærechinus, there was found to be a distinct diminution of fertility. In the most marked instance, obtained in the experiments made on June 2, it was found that when white-spined varieties were fertilised with white-spined, and violet-spined with violet, 98.5 per cent. of the ova reached the blastula stage, and 73 per cent. the eight days pluteus stage. But on cross-fertilising white-spined with violetspined individuals, only 68 per cent. of the ova developed to blastulæ, and 15.6 per cent. to plutei. Also these crossed larvæ were 45 per cent. smaller than the uncrossed. Other series of experiments were made in July, November, and December, the differential fertility seeming to gradually diminish with the progress of the season. Nevertheless, it was always most distinctly present. On crossing the less definitely marked colour varieties of Strongylocentrotus, a small amount of infertility seemed to be present in one series of experiments, but none at all in another.

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As special cases, we have equation (11) reducing the calculation of the force between a circular cylindrical uniform current sheet and a coaxial helical current to the calculation of the coefficients of mutual induction of the helix and the circular ends of the sheet; and the simpler case of the force between a circular cylindrical uniform current sheet and a circular current, which is obtained from the calculations of the coefficients of mutual induction of the circle and the circular ends of the sheet.

I hope that equation (11) may be of service in the accurate calculation of the constants of current weighing apparatus. My attention was drawn to the matter from this point of view in consequence of the Report of the Electrical Standards Committee of the British Association made at Toronto, in which mention is made of the importance of re-determining the ampere.

March 31, 1898.

The LORD LISTER, F.R.C.S., D.C.L., President, in the Chair. Preliminary communications upon the results of the recent Solar Eclipse were made by the following members of the expeditions:

The Astronomer Royal, Sir J. Norman Lockyer, K.C.B., Professor H. H. Turner, Dr. R. Copeland, and Captain E. H. Hills, R.E., and Mr. H. F. Newall.

The Society adjourned over the Easter Recess to Thursday, April 28.

* Professor A. Gray has pointed out to me that this result may be deduced from the consideration that the removal of an element from one end of the sheet to the other is equivalent to a small motion of the sheet parallel to its generating lines. —J. V. J., April 21, 1898.

VOL. LXIII.

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