cleverly constructed by M. Duboscq, recalls the apparatus devised and used by M. E. Becquerel for his important investigations on the phosphorescence of bodies.

Instead of the handle, a wooden pulley of several grooves is substituted, over which a catgut cord passes, which also passes round another pulley of a much larger diameter fixed on the fly-wheel of a gas-engine. This machine, which was very obligingly lent by its inventor, M. Hugon, is of half a horse-power. It works with great regularity; it may be started and stopped almost instantaneously; and unlike steam-engines, the pressure need not be maintained during the times of stoppage. Thus it is of excellent service.

To charge the Leyden battery in which the electricity is condensed, one of Holtz's machines is used, the plate of which is put in motion by the gas-engine. The sparks pass between two metal knobs 11 millims. in diameter. At exactly half the distance between these two knobs is formed the principal focus of the lens of a collimator, so that the luminous rays fall perpendicularly on the vernier. The aperture of the chronoscope is viewed through a magnifyingglass.

Suppose that the electric spark occurs periodically under precisely the same conditions, whilst the mica disk turns almost uniformly. An observer looks through the eyeglass of the telescope and calls out the number of marks which he observes simultaneously with each spark. Another observer registers these numbers, and counts the number of turns which the handle of the chronoscope makes per minute.

Let N be the number of sparks observed, S the total number of marks read, n the number of turns of the handle.

The duration y of the spark, in millionths of a second, is given by the formula

in which μ is a constant parameter, equal to 0.70 for our apparatus. If e denote the angular breadth of the marks on the mica disk, w the angle between the axes of two consecutive marks, and e' the angular breadth of the marks of the vernier, then

Formula (1) assumes N to be a large number; thus, we usually observe series of a hundred sparks.

Other things being equal, the duration of the electric spark is a function of the surface of the Leyden battery, or, in other words, of the number of jars which compose it.

By varying this number x by units from 1 to 9, we found that the duration y may be expressed by the formula

With two zinc knobs, 2.292 millims. apart, the following results were obtained :

It is seen that the difference between the duration observed and the duration calculated has not reached the millionth part of a

second.

With the same zinc knobs 5 millims. apart, we found

Hence the parameter a is independent of the striking-distance.

(6)

(7)

The jars which constitute our battery have an external coated surface of about 1243 square centimetres.

Other physical laws not less important are already brought out by our investigations; more remain to be discovered, and will form the object of subsequent studies. We shall have the honour to communicate to the Academy the results which may be obtained.

These researches were made at the Imperial Observatory at Paris, thanks to the kindness of the Administration, who were good enough to place a spare room at our disposal.-Comptes Rendus, April 25, 1870.

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LONDON:

JOHN VAN VOORST, 1 PATERNOSTER ROW.

1870.

I. On the Cause of the Descent of Glaciers. By JOHN BALL, F.R.S. &c.

....

page 1

II. Reply to M. Delaunay's objection to the late Mr. Hopkins's Method of determining the Thickness of the Earth's Crust by the Precession and Nutation of the Earth's Axis. By Archdeacon PRATT, M.A., F.R.S...

III. On the Path of Electrical Induction- and Disjunction-Currents through Gases of various Densities, and between Poles of different shapes. By E. EDLUND

IV. On a possible Cause of the Bright Line observed by M. Angström in the Spectrum of the Aurora Borealis. By A. S. DAVIS, B.A., Mathematical Master, Leeds Grammar School

V. On the Solution of Linear Partial Differential Equations of the Second Order involving two Independent Variables. By R. MOON, M.A., Honorary Fellow of Queen's College, Cambridge...

VI. On a Simple Method of Constructing high Electrical Resistance. By SAMUEL E. PHILLIPS, Jun.

VII. Researches on the Electrical Discharge. By Professor VON BEZOLD

....

VIII. On the Interchangeability of Heat and Mechanical Action. By the Rev. J. M. HEATH

IX. Proceedings of Learned Societies:

ROYAL SOCIETY:-Mr.WARREN DE LA RUE, Dr. BALFOUR STEWART,
and Mr. B. LOEWY on the Positions and Areas of the Spots ob-
served at Kew during the years 1864-66, also the Spotted Area
of the Sun's visible disk from the commencement of 1832 up to
May 1868; Mr. H. T. BROWN on the Estimation of Ammonia
in Atmospheric Air; Mr. H. E. Roscoe and Dr. T. E. THORPE
on the Relation between the Sun's Altitude and the Chemical
Intensity of Total Daylight in a Cloudless Sky; Prof. C. PIAZZI
SMYTH on Supra-annual Cycles of Temperature in the Earth's
Surface-crust; The Rev. S. HAUGHTON on the Constituent Mi-
nerals of the Granites of Scotland, as compared with those of
Donegal; Mr. H. E. RoscoE on Vanadium..
GEOLOGICAL SOCIETY

X. Intelligence and Miscellaneous Articles :

10

14

383

35

41

42

51

53-68 68-76

Experiments on the Velocity of the Propagation of Sound in Water
in a Cast-iron Conduit 8 decimetres in diameter, by M. Fr. André 76
Experimental Researches on the Duration of the Electric Spark,
by MM. Lucas and Cazin

78

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