12 Things not generally Known. 



the greatest importance ; it is applicable to the elucidation of 

 some of the greatest phenomena of nature. In all these in- 

 stances there is this wonderful tendency in rotation to preserve 

 the axis of rotation unaltered. Prof. Airy's Lect. on Astronomy. 



THE EARTH'S ANNUAL MOTION. 



In conformity with the Copernican view of our system, we 

 must learn to look upon the sun as the comparatively motion- 

 less centre about which the earth performs an annual elliptic 

 orbit of the dimensions and exeentricity, and with a velocity, 

 regulated according to a certain assigned law ; the sun occupy- 

 ing one of the foci of the ellipse, and from that station quietly 

 disseminating on all sides its light and heat ; while the earth 

 travelling round it, and presenting itself differently to it at dif- 

 ferent times of the year and day, passes through the varieties of 

 day and night, summer and winter, which we enjoy. Sir John 

 HerscheVs Outlines of Astronomy. 



Laplace has shown that the length of the day has not varied 

 the hundredth part of a second since the observations of Hippar- 

 chus, 2000 years ago. 



STABILITY OF THE OCEAN. 



In submitting this question to analysis, Laplace found that 

 the equilibrium of the ocean is stable if its density is less than the 

 mean density of the earth, and that its equilibrium cannot be sub- 

 verted unless these two densities are equal, or that of the earth 

 less than that of its waters. The experiments on the attraction 

 of Schehallien and Mont Cenis, and those made by Cavendish, 

 Reich, and Baily, with balls of lead, demonstrate that the mean 

 density of the earth is at least five times that of water, and hence 

 the stability of the ocean is placed beyond a doubt. As the seas, 

 therefore, have at one time covered continents which are now 

 raised above their level, we must seek for some other cause of 

 it than any want of stability in the equilibrium of the ocean. 

 How beautifully does this conclusion illustrate the language of 

 Scripture, " Hitherto shalt thou come, but no further" ! (Job 

 xxxviii. 11.) 



COMPRESSION OF BODIES. 



Sir John Leslie observes, that air compressed into the fiftieth 

 part of its volume has its elasticity fifty times augmented : if it 

 continued to contract at that rate, it would, from its own in- 

 cumbent weight, acquire the density of water at the depth of 

 thirty-four miles. Bat water itself would have its density 

 doubled at the depth of ninety -three miles, and would attain the 

 density of quicksilver at the depth of 362 miles. In descending, 

 therefore, towards the centre, through nearly 4000 miles, the 

 condensation of ordinary substances would surpass the utmost 



