M. de la Place's Mkhanique Celeste. 67 



effects of parallax the small positive quantities that are derived 

 from the remaining seven. Mr. Struve however is inclined 

 to assign 0"*16 of space as the parallax of 8 Ursae Minoris, and 

 0"45 for the sum of the parallaxes of a Cygni, and i Ursae 

 Majoris. His learned coadjutor, M. Walbeck, who, it appears, 

 has undertaken the calculations, is disposed to attribute the 

 greatest portion of this parallax to the smaller star : a circum- 

 stance so improbable requires very strong evidence for its 

 support. 



But whatever reasonable doubt we may entertain as to any 

 one given result * relating to such extremely minute quantities, 

 yet the mean of the whole must be admitted to deserve very 

 great confidence ; and it is to this view of the subject (omitted 

 by the learned author) that I wish to direct the attention of 

 astronomers. 



If we take the mean of the fourteen results as relating ge- 

 nerally to stars from the 1st to the 4th magnitude, it will ap- 

 pear that the meafl sum of the parallaxes of two opposite stars 

 is equal to 0'.036 of space, or the parallax of a single star 

 equal to 0.018. 



If any reliance can be placed on these observations, every 

 attemptto determine the parallax of these stars in declination 

 must be entirely hopeless ; since in this case we can only 

 measure the shorter axis of the ellipse, and the uncertainty of 

 refraction must amount at least to twenty times the quantity 

 we are in search of. — Qtcart. Journ. of Science, vol. xvi. p. 365. 





m. de laplace's great work. 

 The fifth and last volume of the MechaniqueCelestehasm&de 

 its appearance, in which the question of the form of the earth 

 is discussed in various new points of view: namely — 1st, The 

 dynamic effect of the presence and distribution of the waters 

 on the surface of the globe. 2dly, The compression to which 

 the interior beds are subjected. 3dly, The change of size, 

 which may result from the progressive cooling of the earth. — 

 The author has arrived at the following results: That the 

 great mass of the earth is by no means homogeneous ; that 

 the beds situate at the greatest depth are the most dense ; that 

 those beds are disposed regularly round the centre of gravity 

 of the globe, and that their form differs little from that of a 

 curved surface generated by the revolution of an ellipsis ; that 

 the density of water is nearly five times less than the mean 

 density of the earth ; that the presence and distribution of the 



* It should be remembered, that in a scries of observations, it generally 

 happens that some results will be erroneous by a greater quantity than the 

 mean probable error. 



I 2 waters 



