MODERN SCIENCE. 203 



to be applied at stations where Halley's method cannot 

 be used, and which consists in noting the time of the 

 beginning of the transit at a station in the North where 

 it begins earliest, and at another station in the South 

 where it begins latest. These details are given because 

 of the importance of Venus's transit and the great interest 

 attached to the latest transits, those of 1874 and 1882, of 

 which most of us have heard. Halley found also a simple 

 formula to measure the ALTITUDE OF MOUNTAINS with the 

 barometer the mercury sinking one inch in 900 feet ; 

 studied the variations of the compass ; explained the causes 

 of the inequality of level in the Mediterranean Sea to be due 

 to evaporation. He invented the MERCURY THERMOMETER 

 (1680). 



1692 1762. Bradley discovered the phenomenon called 

 the ABERRATION OF FIXED STARS an apparent displacement 

 of celestial bodies due to the progressive motion of light and 

 the orbital motion of the earth the aberration depending 

 upon the ratio between the velocity of light and of the 

 earth's motion. This phenomenon is sometimes called 

 "aberration of light" but rather incorrectly. Bradley 

 discovered and explained the NUTATION (oscillation) of the 

 earth's axis that is, the small gyration of the axis around 

 the mean position due to the precession of the equinoxes. 

 This motion of nutation takes place in about nineteen years 

 in a small ellipse. These two discoveries are of high 

 importance in the study of astronomy. 



17361813. Lagrange demonstrated, with Poisson, 

 Laplace's theory of the INVARIABILITY OF THE GREAT AXES 

 of the planetary orbits ; solved the problem of the libration 

 of the moon that is, he explained with Laplace, with whom 

 he worked hand in hand, that the moon always presents the 

 same side to us because the globe is not a sphere, but an 

 ellipsoid, i.e., it has its equator elongated towards the earth 

 this elongation being due no doubt to the tide of matter (now 

 solidified) which was caused at one time by the earth's 

 powerful attraction. By this elongation, the moon's earth- 

 ward hemisphere contains an excess of matter which, by the 

 earth's attraction the earth having eighty times more matter 



