64 Britain's Heritage of Science 



Almanac, which has proved to be of immeasurable value 

 to all seamen. Maskelyne remained its editor until his 

 death. He also re -organized in many ways the work and 

 instrumental equipment of the Greenwich Observatory, and 

 instituted an important research which led to the first 

 determination of the density of the earth. To appreciate 

 the importance of this experiment, we must remember 

 that by noting the rate of fall of a body we can measure the 

 force with which the earth attracts it, but not knowing the 

 total mass of the earth, we cannot tell how much one pound 

 of matter would attract another pound at a given distance. 

 That can only be ascertained by measuring the attraction 

 between masses both of which are known. From the result 

 of such a measurement the mass of the earth may be calcu- 

 lated, and as its dimensions are known, we can deduce its 

 mean density. The problem of finding the density of the 

 earth is, therefore, identical with that of finding the gravita- 

 tional attraction between known masses, and herein lies 

 its chief value. Maskelyne 's method consisted in deter- 

 mining the deflexion of a plumb line in the neighbourhood 

 of a mountain. As this deflexion cannot be observed directly, 

 we must have recourse to an indirect method; but this 

 presents no difficulties. If the latitudes of two places, one 

 to the north and the other to the south of a mountain, 

 be determined astronomically, and their distances directly 

 measured, the discrepancy between the observed and 

 measured differences of latitude gives us the data we want 

 for calculating the gravitational effect of the mountain. 

 The method cannot give very accurate results, as the density 

 of the material composing the mountain must be taken into 

 account, and this requires a geological survey and complicated 

 calculations. Maskelyne was assisted in his measurements, 

 which were conducted in the neighbourhood of the mountain 

 Schehallien in Perthshire, by Charles Hutton (1737-1823), 

 Professor of Mathematics at the Military Academy, Wool- 

 wich; the figures they obtained showed that bulk for bulk 

 the material of the earth is on the average between 4*48 and 

 5 -38 times heavier than water. 



While learning at Oxford and Cambridge rapidly declined 

 after the first impulse of Newton's discoveries had died away, 



