114 On the Figure of the Earth. 



if the day has increased. Tlie cojistancy of the lunar month will 

 indicate tlie invariability of the day. 



All observations combine to prove that from the time of the 

 Chaldeans, to our own days, the duration of the lunar month 

 has been gradually diminishing. It follows, therefore, from 

 what has been stated, either that the velocity of the moon has in- 

 creased, or that the solar day has lengthened. But M. de Laplace 

 has discovered by theory, that there is in the motion of the 

 moon, an inequality known by the name of secular equation, 

 which depends on the variation of the excentricity of the earth's 

 orbit, and of which the value in each century may be de- 

 duced from the change of this excentricity. By the assistance 

 of this equation, the increase of velocity above noticed is per- 

 fectly accounted for. There is, therefore, no reason to suppose 

 that the duration of the day is not sensibly constant. 



Let us admit for a moment, with M. de Laplace, that this dura- 

 tion, surpasses at present that of the time of Hipparchus, by the 

 hundredth of a decimal second. The duration of a century now, 

 or of 36,525 solar days, would be longer than the duration of a 

 c9ntury 2,000 years ago, (Hipparchus lived about 120 years before 

 our era), by 365."25. In this interval of time, the moon describes 

 an arc of 534".6; this quantity, therefore, expresses the difference 

 between two arcs traversed by the moon in a century now, and 

 in one of the time of Hipparchus ; but as these arcs, determined 

 by observation and corrected by the secular equation, do not 

 differ by a quantity so large, we may conclude tliat in this long 

 interval the duration of the day has not varied by the hundredth 

 of a second. 



Annates de Chimie, xi. p. 31. 



Art. IX. On the Preparation of Oxygenated Water. By 

 M. Thenard. 



The preparation of oxygenated water requires certain precau- 

 tions, without which success will only be partial. That none 

 may be omitted, I shall describe the process in the most minute 

 manner. 



