154: ANNUAL OF SCIENTIFIC DISCOVERY. 



While we are surrounded on every side by the monuments of violent 

 volcanic convulsions, we possess no record of the velocity of the axial 

 rotation of our planet in antediluvian times. It is of the greatest im- 

 portance that we should have an exact knowledge of a change in this 

 velocity, or in the length of the day during historic times. The inves- 

 tigation of this subject by the great Laplace forms a bright monument 

 in the department of exact science. 



These calculations are essentially conducted in the following man- 

 ner : In the lirst place, the time between two eclipses of the sun. wide- 

 ly apart from each other, is as accurately as possible expressed ia 

 davs. and from this the ratio of the time of the earth's rotati.::i to the 



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mean time of the moon's revolution determined. If, now, the obser- 

 vations of ancient astronomers be compared with those of our present 

 time, the least alteration in the absolute length of a day may IK- detect- 

 ed by a change in this ratio, or in a disturbance in the lunar re\ oluii.;:!. 

 The most pei feet agreement of ancient records on the movements of 

 the moon and the planets, on the eclipses of the sun, &c. revealed to 

 Laplace the remarkable fact that, in the course of 25 centuries, the time 

 in which our earth revolves on its axis has not altered -^-^ part of a sex- 

 agesimal second ; and the length of a day therefore may be considered 

 to have been constant during historic times. This result, as impor- 

 tant as it was convenient for astronomy, was nevertheless of a nature 

 to create some difficulties ibr the physicist. ' With apparently good 

 reason it was concluded that, if the velocity of rotation had remained 

 constant, the volume of the earth could have undergone no change. 

 The earth completes one revolution on its axis in 8G,-iOO sidereal sec- 

 onds ; it consequently appears, if this time has not altered during 

 2,500 years to the extent of -$\-$ of a second, or -o'ToVTo'o" P art ^ a 

 day, that during this long space of time the radius of the earth also 

 cannot have altered more than tiiis fraction of its length. The earth's 

 radius measures G,oG ( J,800 meters, and therefore its length ought not 

 to have diminished more than 15 centimeters i:i 25 centuries. 



The diminution in volume, as a result of the cooling-process, is 

 however, closely connected with the changes on the earth's surface. 

 When we consider that scarcely a day passes without the occurrence 

 of an earthquake or shock in one place or another, and that of the 

 300 active volcanos some are always in action, it would appear 

 that such a lively reaction of the interior of the earth against the 

 crust is incompatible with the constancy of its volume. This apparent 

 discrepancy between Cordiers theory of the connection between the 

 cooling of the earth and the reaction of the interior or the exterior 

 parts, ami Laplace's calculation showing the constancy of the length 

 of the day, a calculation which is undoubtedly correct, has induced 

 most scientific men to abandon Cordier'.s theory, and thus to deprive 

 themselves of any tenable explanation of volcanic activity. 



The continued cooling of the earth cannot be denied, for it takes 

 place according to the laws of nature ; in this respect the earth cannot 

 comport itself differently from any other mass, however small it be. 

 In spite of the heat which it re -eives from the sun, the earth will have 

 a tendency to cool so long as the temperature of its interior is higher 

 than the mean temperature of its surface. Between the tropics the 

 mean temperature produced by the sun is about 28, and the sun 



