FIGURE OF THE EARTH. 315 



to the measurcmeut of the arc of 1° 11' 30" comprised hetween Alkmaar and 

 Bergen-op-Zoom ; but the result of his uudei'taking, calculated and discussed 

 by Muschembrock, did not see the light until a later period, when others had 

 been oljtaiued of the same kind with higher pretensions to certainty. 



In like manner with SchncU, Norwood determined, in lOoO, the difference of 

 latitude between York and London, equal to 2^ 28', by the difference of the al- 

 titude of the sun between their respective horizons at the period of the sol- 

 stices ; and afterwards measuring the distance between those cities, he arrived at a 

 valuation, too great however, of the length of a degree of the meridian; making 

 it 57,442 toises, or about 111,955 metres. 



For its novelty, if nothing else, there should be mentioned in connexion 

 with the preceding attempts the method proposed by Manrolico, at that epoch 

 of measurements, to determine the terrestrial radius. Assuming the unc|uestion- 

 able fact that the extension or breadth of land which, seen from the sea-shore, 

 or in the interior of a nearly level cotintry, depends at once on the height at 

 which the spectator is stationed, and on the curvature or radius of the earth, 

 Manrolico thought that by measuring the height of a mountain near the sea 

 and the route traced without change of direction by a bark until it disappears 

 below the horizon, the value of the radius sought might be deduced, without 

 reference to, any astronomical observation. This process, put in practice at a 

 later period, with some variations, and only by way of trial, has led to a result 

 greater than might have been expected, being complicated with some causes of 

 error and uncertainty ; we do not know that the solution of the problem was 

 ever attempted in the lifetime of its author. 



Another idea, ingenious like all proceeding from the same source, occurred to 

 Kepler, and Riccioli undertook to realize it, although in practice, from the im- 

 perfection of instruments among other considerations, it could not but lead to 

 a result very distant from the true one. The idea consists in measuring upon 

 any given surface of ground the greatest lineal distance possible, and then cal- 

 culating the angles of the two respective verticals with the common line of 

 vision comprised between the extremes of the base. It requires but a slight 

 notion of geometry to comprehend how delicate was the operation which Ric- 

 cioli took charge of, and how little reliance could be placed on results deduced 

 from such a process. 



These different estimates — for they merit no other name — towards ascertaining 

 the magnitude of the earth, were but the prelude to other more exact processes, 

 and show the necessity that was felt, but 200 years ago, of obtaining a pre- 

 cise knowledge of the dimensions of our planet, as well as the oblivion into 

 which the labors of antiquity had fallen or the small importance attached to 

 them. In proof of this, let us remember that at the end of the fifteenth century and 

 beginning of the next, Columbus shaped his coiu-se towards the unknown shores 

 of India and Magallanes traced his adventurous progress across the Pacitic, 

 upon the delusive supposition that the earth was of much less size than it really 

 is; and that, in the midst of the seventeenth century, Newton himself, in 

 whom the highest genius was not at variance with extensive erudition and a 

 sound judgment, found himself under the necessity of suspending his researches 

 respecting the reciprocally attractive action of the earth and the moon, in con- 

 sequence of the want of an approximate valuation for the radius of our globe. 

 So pressing did the necessity referred to appear, that when the Academy of 

 Sciences of Paris was instituted, in 1666, one of its first acts was to commit to 

 Picard, a distinguished member of that learned assembly, the measurcnieut of 

 a new arc of the meridian ; a Avork which this astronomer completed before the 

 end of 1670, by the method adopted by Eratosthenes and Posidouius, as well 

 as by Schuell and Norwood, but which was executed with so much accuracy 

 in the details as to form an epoch in the annals of astronomy and geodesy. 



