460 LECTURE XLVIII. 



graphers and our seamen, before Norwood had measured the earth, that 60 

 English miles were contained in one degree of latitude on the surface of the 

 earth. But as this is a very faulty supposition, each degree containing 

 ahout 69^ of our miles, his computation did not answer expectation ; whence 

 he concluded that some other cause must at least join with the action of 

 the power of gravity on the moon. On this account he laid aside for that 

 time any further thoughts upon this matter. But some years after, a letter 

 which he received from Dr. Hooke, put him on inquiring what was the real 

 figure, in which a body let fall from any high place descends, taking the 

 motion of the earth round its axis into consideration. Such a body, having 

 the same motion, which by the revolution of the earth the place has from 

 whence it falls, is to be considered as projected forwards, and at the same 

 time drawn down to the centre of the earth. This gave occasion to his 

 resuming his former thoughts concerning the moon ; and Picart, in France, 

 having lately measured the earth, by using his measures, the moon appeared 

 to be kept in her orbit purely by the power of gravity ; and consequently, 

 that this power decreases as you recede from the centre of the earth, in the 

 manner our author had formerly conjectured. Upon this principle he 

 found the line described by a falling body to be an ellipsis, the centre of 

 the earth being one focus. And the primary planets moving in such orbits 

 round the sun, he had the satisfaction to see, that this inquiry, which he 

 had undertaken merely out of curiosity, could be applied to the greatest 

 purposes. Hereupon he composed near a dozen propositions relating to the 

 motion of the primary planets about the sun. Several years after this, 

 some discourse he had with Dr. Halley, who at Cambridge made him a 

 visit, engaged Sir Isaac Newton to resume again the consideration of this 

 subject ; and gave occasion to his writing the treatise which he published 

 under the title of Mathematical principles of natural philosophy. This 

 treatise, full of such variety of profound inventions, was composed by him, 

 from scarce any other materials than the few propositions before men- 

 tioned, in the space of one year and a half." 



The astronomers of Great Britain have not been less diligent in the prac- 

 tical, than successful in the theoretical part of the science. The foundation 

 of the observatory at Greenwich was laid in 1675, some years before the 

 completion and publication of the discoveries of Newton. It is with the 

 erection of this edifice that the modern refinements in practical astronomy 

 may be said to have commenced ; its immediate object was to assist in the 

 perfection of the science of navigation, and the series of observations, which 

 have been made in it, has afforded an invaluable fund of materials to 

 astronomers of every country. A reward had been proposed, more than 

 half a century before, by Philip the Third, of Spain, for the discovery of a 

 mode of determining the longitude of a ship at sea ; and the states of Hol- 

 land had followed his example ; a large reward was also offered by the 

 French government in the minority of Louis the Fifteenth. In 1674, 

 Mr. St. Pierre, a Frenchman, had undertaken to determine the longitude 

 of a place from observations of the moon's altitude, and King Charles the, 

 Second had been induced to appoint a commission to examine his propo- 

 sals. Mr. Flamsteed was consulted by the commissioners, and was added to 



