DENSITY OF THE EARTH. 23 



latitude, requires great exactness in the observed differences 

 of longitude. Cassini de Thury and Lacaille, as early as 

 1740, availed themselves of powder signals in the measure- 

 ment of an arc perpendicular to the meridian in the parallel 

 of Paris. At a more modern period, the length of parallel 

 arcs, and the differences of longitude, were measured with 

 far better instrumental means, and with greater certainty, in 

 the course of the great Trigonometrical Survey of England ; 

 the determinations were between Beechy Head and Dunnose, 

 and between Dover and Ealmouth,( 8 ) the differences of 

 longitude being indeed only 1 26' and 6 22' respectively. 

 The most brilliant operation of this kind was undoubtedly 

 the measurement of the arc between the meridians of 

 Marennes on the west coast of France, and Fiume. It 

 crosses the most western chain of the Alps, and the 

 Lombard plains of Milan and Padua : the measurement was 

 executed by Brousseaud and Largeteau, Plana and Carlini, 

 and extends over a direct distance of 15 32' 21", almost 

 entirely under the middle parallel of 45. The many pen- 

 dulum experiments which were made in the neighbourhood 

 of the mountains confirmed in a remarkable manner the 

 previously recognised influence of local attraction, shown 

 by the comparison of the astronomical latitudes with the 

 results of the geodesical measurements.^) 



Next to these two classes of direct measurements of de- 

 grees, (a) of meridian al, and (b] of parallel arcs, mention 

 should be made of a purely astronomical mode of determining 

 the Earth's figure. It is founded on the influence exercised 

 by the Earth on the motion of the Moon (i. e. on the 

 inequalities in her latitude and longitude). Laplace, who 

 first recognised the cause of these inequalities, also indicated 



