222] The Shape of the Earth 279 



ellipticity were deduced, the discrepancies being partly due 

 to different theoretical methods of interpreting the results 

 and partly to errors in the arcs. 



A measurement, made by Jons Svanberg ([771-1851) in 

 1801-3, of an arc near that of Maupertuis has in fact 

 shewn that his estimate of the length of a degree was 

 about 1,000 feet too large. 



A large number of other arcs have been measured in 

 different parts of the earth at various times during the 

 1 8th and igth centuries. The details of the measurements 

 need not be given, but to prevent recurrence to the subject 

 it is convenient to give here the results, obtained by a 

 comparison of these different measurements, that the 

 ellipticity is very nearly ^J^, and the greatest radius of the 

 earth (c A in fig. 78) a little less than 21,000,000 feet or 

 4,000 miles. It follows from these figures that the length 

 of a degree in the latitude of London contains, to use Sir 

 John Herschel's ingenious mnemonic, almost exactly as 

 many thousand feet as the year contains days. 



222. Reference has already been made to the supremacy 

 of Greenwich during the i8th century in the domain of 

 exact observation. France, however, produced during this 

 period one great observing astronomer who actually accom- 

 plished much, and under more favourable external conditions 

 might almost have rivalled Bradley. 



Nicholas Louis de Lacaille was born in 1713. After he 

 had devoted a good deal of time to theological studies 

 with a view to an ecclesiastical career, his interests were 

 diverted to astronomy and mathematics. He was intro- 

 duced to Jacques Cassini, and appointed one of the 

 assistants at the Piris Observatory. 



In 1738 and the two following years he took an active 

 part in the measurement of the French arc, then in process 

 of verification. While engaged in this work he was ap- 

 pointed (1739) to a poorly paid professorship at the 

 Mazarin College, at which a small observatory was erected. 

 Here it was his regular practice to spend the w r hole night, 

 if fine, in observation, while " to fill up usefully the hours 

 of leisure which bad weather gives to observers only too 

 often " he undertook a variety of extensive calculations and 

 wrote innumerable scientific memoirs. It is therefore not 



