202 PROGRESS OF SCIENCE. 



January because in July we are further removed from Jupiter 

 by 190 millions of miles; and Roemer consequently found 

 light to travel at the rate of some 190 thousand miles per 

 second, and to reach us, roughly speaking, in 8 min. 18 sec. 

 from the sun that is, nine million times faster than an express 

 train or ten thousand times the velocity of the earth. (See 

 Foucault and Fizeau.) Roemer's discovery was a much more 

 conclusive proof of the motion of the earth around the sun 

 than the calculation of an almost imperceptible parallax (the 

 apparent change of position of a heavenly body in conse- 

 quence of its being viewed from different parts). 



16461719. Flamsteed drew up a TABLE OF THE 

 TIDES, and a catalogue of fixed stars of remarkable accuracy. 

 His observations on the moon were inserted in Newton's 

 Principia. He proved the falsehood of astrology by his 

 Ephemerides. 



1656 1742. Halley went to St. Helena (1676) to study 

 the heavens, and, there, observed the TRANSIT OF MERCURY, 

 and catalogued three hundred and fifty stars in the SOUTHERN 

 HEMISPHERE ; demonstrated the periodical return of comets 

 by his earnest observation (1682) of the one that bears his 

 name, predicting its return in 1758, and every seventy-six 

 years ; pointed out the Secular Inequality of the moon and 

 of the satellites of Jupiter and Saturn ; from his observation 

 of the transit of Mercury, and the time it took, he was able 

 to make (1691 and 1716) the happiest suggestions for the 

 measurement of the sun's distance from us, by means of the 

 transit of Venus a phenomenon which seldom occurs (two 

 coming in eight years, and then only one hundred and 

 twenty-two years after). He left accurate instructions to 

 accomplish that object during the next transit in 1761. 

 The beautiful method he indicated, and which consisted 

 in observing the duration of the transit from two or more 

 stations far apart, east and west, was followed with the 

 best result. Several expeditions, sent out in 1761 and 1769, 

 enabled astronomers to approximate the distance the 

 distance found being 91,368,000 miles, or 108 times the 

 sun's diameter. We may remark that J. N. Delisle (1688 

 1768) proposed another method of observing Venus's transit, 



