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The periodicity of the phenomenon may manifest itself again in the renewal of the shocks. But the maxima and minima of frequency will not correspond with the passage of the principal seismic waves. The order may even be wholly reversed. It is a phenomenon analogous to the "establishment of the port" in the oceanic tides.

The physical law, that earthquakes are more numerous at the syzygies than at the quadratures, is one that is verified by simply counting the days of earthquakes during a sufficiently large number of years. This I have done for a period of 50 years, from 1801 to 1850,' and also for each half of this period. Again, dividing these 50 years into 10 periods of 5 years each, I have detected two maxima and two minima in nine of these partial periods. In seven, the maxima has occurred at the syzygies and the minima at the quadratures; in two the reverse has taken place. The principal, above pointed out, of the establishment of the port is alone sufficient to explain this apparent anomaly. In these two 5-year periods, there was a series of local shocks in a region where earthquakes are unfrequent.

The quinquennial period from 1810 to 1815 affords no sensible maxima and minima. But the facts on record are few. During the unhappy years of 1814, 1815, the journals took little note of subterranean commotions.

Prof. Perrey has made out, from the facts which he has collected, for the first half of the present century (from 1801 to 1850), that there were 5388 lunar days on which earthquakes occurred; or, counting as so many separate days, where 2 or 3 or more earthquakes occur on the same day, but in distant countries and wholly independent, (the most correct method for his calculations,) 6596 lunar days. In order to refer these days to the syzygies and quadratures, he divides the mean lunation of 29.53 days into 8 equal parts, and then groups these into 4, by uniting the 1st and 8th and 4th and 5th, for the new and full moon or syzygies, and the 2nd and 3d and 6th and 7th for the quadratures.

Arranging thus the phenomena, he obtained for the 5388 days,-276148 at the syzygies and 2626 52 at the quadratures, leaving a difference in favor of the syzygies

of 134.96.

For the 6596 days, he obtained 3434-64 at the syzygies and 3161-36 at the quadratures, leaving 273-28 in favor of the syzygies.

In a similar manner, for the half century preceding, or from 1751 to 1800, he obtained 1901-18 earthquake days at the syzygies, and 1753-82 at the quadratures, the difference in favor of the syzygies being 147.36.

Counting the earthquake days during the years 1761 to 1800, which occur within the period of five days, from the second day before the apogee and perigee to the second day after inclusive, he found 526 earthquake days at the perigee and 465 at the apogee, leaving a difference of 60 in favor of the perigee; or leaving off the outer two of the five days, the result was 3134 at the perigee and 278 at the apogee, or an excess of 35 at the perigee.

Taking the earthquakes of Reggio in Calabria as given for the years 1836 to 1853 (18 years) in a Journal kept by M. S. Arcovito, he finds 437 earthquake shocks at the syzygies and 349 at the quadratures, or an excess of 88 at the syzygies. He also obtains, for the number of shocks when the moon was less than 45° from the meridian 413, when more than 45°, 347, leaving 66 shocks in favor of the former.See for a fuller statement of these results, and additional facts, Comptes Rendus, lii, 146-151, Jan. 28, 1861.

We have above supposed the central nucleus of the earth to be in an incandescent state, either liquid or viscous. But can this be without the existence of intense chemical action proportional to the high temperature of incandescence? Under such circumstances would there not be electro-magnetic currents? May it not be that, through the influence of such currents, which Dr. Ami Boué makes the first cause of earthquakes, and also under that of the various chemical actions going on, gases would be developed so as to form a more or less continuous atmosphere between the central nucleus and the crust? And should not the presence of these gases modify in some way, the dynamic action of the earthquake waves? Is not their sudden explosion, the cause, at times, of transient disturbances in the central mass? And, consequently, are there not thence sensible reactions against the inner surface of the crust, causing strong vibrations that are propagated to the outer surface?

This idea, which I have elsewhere brought forward, is remarked upon as follows by the learned author of the Histoire des Progrès de la Geologie. "As to these immense tempests which the author raises at the surface of the incandescent fluid, whose waves of fire beat against the flanks of the mountains which project downward like gigantic stalactites, they appear to us to be a little remote from the domain of science and to pertain rather to that of the imagination."

But, without taxing too much the imagination, can we not see that these chemical actions, which others have made the sole cause of earthquakes, may produce some perturbations, or modifications, in earthquake movements which shall obscure at times. the periodicity?

Formerly, especially during the last century, the existence of numerous vast caverns in the earth, for the propagation of earthquakes, was admitted. We do not deny the existence of such caverns; but, in our view, instead of their favoring earthquake vibrations they would arrest, or at least impede, them. The simplest break will modify the rate and direction of the undulations. But such caverns should also cause, in some cases, molecular vibrations which, on being propagated to the earth's surface, would not differ from ordinary earthquakes. The liquid matter, in entering the cavities, would also cause shocks of a similar kind. Hence may come some of those facts registered in earthquake tables, which interfere with the exhibition of the periodicity.

We pass by other causes to which earthquakes have been attributed. Several, although less general than they have been supposed to be, may be admitted among special or secondary causes.

• Memoir on the earthquakes of the Scandinavian peninsula, Voyages de la Commission Scientifique du Nord en Scandinavie, en Laponie, etc., Paris, 1845.

It cannot be too often repeated, that earthquakes are not of one single kind, identically the same. They are various both in causes and effects; I aim simply to bring out in relief the principal cause. I seek to establish its truth, by the differential influence manifested in its march as regards time.

As to the geographical relations of earthquakes, I say only, that no region is secure from subterranean movements; and that no geological formation is exempt; but that the mountain systems of the surface appear to exercise a great influence at least on their propagation if not on their frequency. The vibrations are usually propagated along the main axis of a chain; as has been observed in the Pyrenees and the Andes. In the great valleys occupied by rivers, the mean direction, as calculated by Lambert, appears to be that of the course of the depression. I have shown this to be the fact with the basins of the Rhone and the Rhine, where the direction is nearly meridional, and the basin of the Danube, which has a transverse course, or from west to east.

In France, the departments most subject to earthquakes appear to be those about the mouths of the large rivers. The department of the Isère, where the depression of the valley of the Rhone forms a kind of node with that of the Saône, is the only one which can compare with the kind just mentioned in number of earthquakes.

It is a question whether or not a double orthogonal curvature in the outer surface indicates an analogous structure through the whole thickness of the crust; and whether or not a structure of this kind presents less resistance to the propagation of shocks.

Whatever may be the cause of the molecular vibration at any given point in the crust, vibrations will be propagated in the form of waves; and in a homogeneous medium, the waves will be spherical and concentric. How will it then be in a medium which is not homogeneous, or is of unequal density? This cannot be decided without investigation.

In the case of the propagation of a series of waves which succeed one another through each point in the sphere of undulation and make successive shocks at the earth's surface, the shocks directly over the centre or focus of the vibrations will be vertical: and the obliquity, or variation from verticality, will be greater the more remote the place of emergence at the surface is from the centre of vibration alluded to; or, the locality being fixed, the nearer this centre is to the surface.

There can be no rotary shocks; the cases of apparent rotation we have explained elsewhere. But does the direction of a shock indicate the point from which it actually comes? I believe not. The difference in the rocks encountered should produce derivative and reflected undulations, as in the case of waves of sound.

Breaks in the rocks, as the caverns referred to, must modify their propagation, vary their direction and weaken their intensity, and may extinguish them; and this may account for the simultaneous shaking of two regions while an intermediate locality is undisturbed a phenomenon of so frequent occurrence in certain parts of America that the people speak of it under the expression of the earth being bridged within, or suspended.

Boussingault recognized, as the principal cause of the earthquakes of the Andes, the continual and progressive sliding of the dislocated rocks of which they consist; and he considered the phenomenon as incessant in South America, an earthquake taking place, in his view, somewhere in the Andes at every instant of time.

These views are not at variance with my own. Any slidings due to gravity will be caused, or favored, by the daily vibrations whose effects and causes have been considered.

Calculation demonstrates the existence of two kinds of waves moving with different velocities around a centre of vibration; I admit readily, with Mr. Wertheim, the coëxistence of these two kinds of waves. If then there are several successive sets of vibrations at a given point, each will propagate the double system of waves. It will be the same, also, if there are simultaneous disturbances at a number of neighboring points. The waves of greatest velocity of one set will overtake and pass by those of least velocity in the preceding set, and at an interval of distance depending on the interval of time between the successive vibrations.

The ingenious idea that two species of waves or undulations pertaining to two successive sets of vibrations may produce at the surface of the earth one vibration of combined intensity, has nothing in it to which I can object. It is analogous to the interference of waves of light. We also admit, with Mr. Wertheim, that two such combined waves may occasion greater violence of disturbance than the passage of two successive waves. In this case, the surface of the earth under vibration, if perfectly homoge neous, should present concentric zones in which the disturbance will be alternately more and less great. I would say, however, that I do not believe that such an alternation of effects from earthquakes has ever been observed. For such results, not only would a uniformity in the earth's crust be required, but also the structures on the surface to be upset or damaged should have an identity of construction and of position with reference to the points of compass which cannot be looked for.

At some future time, I propose to consider, from this point of view, the occurrence of the first shock more or less light which precedes often the great shakings, and of the harmless vibrations which separate the disastrous shocks; and also the short interval AM. JOUR. SCI.-SECOND SERIES, VOL. XXXVII, No. 109.—JAN., 1864.

of relative repose or simple tremulousness which separates two consecutive shocks of moderate intensity.

As to the velocity of the propagation of shocks, we make no definite statement. Notwithstanding the trials of Dr. Julius Schmidt, we have no confidence in the results derived from his calculations, believing that they are based on too uncertain data.

The methods proposed by Mr. R. Mallet, will we doubt not, if carried out, give an exact determination of this element in the phenomena of earthquakes. We recognize the importance of the problem. But even if the means of noting time should be much better than at present, and in more general use, it may be doubted whether numerical results will be obtained of much value to science.

ART. II. The Classification of Animals based on the principle of Cephalization; by JAMES D. DANA.-No. II. Classification of Insects.1

THE principles which have been presented in my former article on the classification of animals may be further exemplified by a discussion of the natural system of classification in a few subdivisions of the animal kingdom; and at the present time I take up for this purpose the order of Insects.

The subject may be appropriately introduced by a recapitulation, arranged so as to be convenient for reference, of those of the characteristics bearing on grade which are of most prominent importance. In connection with the mention below of these 'characteristics, the number of the page is added on which they are explained and illustrated in the preceding volume of this Journal. Other characteristics not here enumerated will be found on the pages referred to.

Under each head the characteristic to be looked for in a superior group is first mentioned; and then those of related kinds in inferior groups.

I. In a superior group, (A) a prosthenic condition. In an inferior group (B) a metasthenic condition of different grades or kinds; or in a still lower group (C) a urosthenic condition. (P. 323.)

These conditions come under the transferent method of cephalization, which is exhibited in a transfer of force and function towards the head (preferent) with ascending grade, or in the reverse direction (retroferent) with descending.

This transfer is similar in nature to that which results in amplificate forms and the reverse; in one direction, the descending, it is outward or

1 For Article I, see last volume of this Journal, p. 321.

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