220, a i] The Density of the Earth 275 



The Cavendish experiment, as it is often called, has 

 since been repeated by various other experimenters in 

 modified forms, and one or two other methods, too technical 

 to be described here, have also been devised. All the 

 best modern experiments give for the density numbers 

 converging closely on 5|, thus verifying in a most striking 

 way both Newton's conjecture and Cavendish's original 

 experiment. 



\Vith this value of the density the mass of the earth is 

 a little more than 13 billion billion pounds, or more 

 pi ' isely 13,136,000,000,000,000,000,000,000 Ibs. 



2^0. While Greenwich was furnishing the astronomical 

 wond with a most valuable series of observations, the Paris 

 Observatory had not fulfilled its early promise. It was 

 in fact suffering, like English mathematics, from the evil 

 effects of undue adherence to the methods and opinions of 

 a distinguished man. Domenico Cassini happened to hold 

 several erroneous opinions in important astronomical 

 matters ; he was too good a Catholic to be a genuine 

 Coppernican, he had no belief in gravitation, he was firmly 

 persuaded that the earth was flattened at the equator instead 

 of at the poles, and he rejected Roemer's discovery of the 

 velocity of light. After his death in 1712 the directorship 

 of the Observatory passed in turn to three of his descendants, 

 the last of whom resigned office in 1793; and several 

 members of the Maraldi family, into which his sister had 

 married, worked in co-operation with their cousins. Un- 

 fortunately a good deal of their energy was expended, first 

 in defending, and afterwards in gradually withdrawing from, 

 the errors of their distinguished head. Jacques Cassini, for 

 example, the second of the family (1677-1756), although 

 a Coppernican, was still a timid one, and rejected Kepler's 

 law of areas ; his son again, commonly known as Cassini de 

 Thury (1714-1784), still defended the ancestral errors as 

 to the form of the earth ; while the fourth member of the 

 family, Count Cassini (1748-1845), was the first of the 

 family to accept the Newtonian idea of gravitation. 



Some planetary and other observations of value were 

 made by the Cassini-Maraldi school, but little of this work 

 was of first-rate importance. 



221. A series of important measurements of the earth, 



