90 LECTURES 



in whicli we shall have known two angles and the included side. 

 Adding the distances A M, M N, N 0, F, and P B, we shall have 

 the entire distance from A to B measured along the continued ocean 

 level. It is thus that science, hy her magic touch, levels down the 

 mountains and fills up the valleys, and makes for herself a path 

 which, we may truly say, the vulture's eye hath not seen. 



To solve the problem we have in hand, and determine definitely 

 the figure and magnitude of the earth, we must have the results of 

 measurements in different latitudes. 



We shall briefly refer to some of the principal measurements which 

 have been executed before we deduce from them the exact dimensions 

 of the earth. 



From the early part of the ninth century, when the Arabian astrono- 

 mers, as before stated, measured an arc of the meridian on the plain 

 of Mesopotamia, this question seems to have been lost sight of for 

 more than six hundred years. 



1. In 1528, Fernel, an eminent physician of Paris, revived the 

 problem by measuring the distance from Paris to Amiens, in Picardy, 

 by counting the revolutions of his coach wheels. 



2. Nearly a hundred years later, (1617,) Snellens, of Leyden, 

 measured an arc of the meridian, in Holland, between Alkmaer and 

 Bergen-op-Zoom. He was the first to adopt the principle of trian- 

 gulation. 



3. Some few years after the measurement of Snellens, the English 

 astronomer, Norwood, (1635,) measured an arc between London and 

 York. He measured parts of the distance with a chain, other parts 

 by pacing, and where the ground was rough he estimated the distance 

 as well as he could. 



The results of these measurements, on account of the imperfection 

 of methods and instruments, are of little or no value. They only 

 indicate the growing interest in the problem. 



4. Picard's measurement. 



It was not until 1G69 that an arc of the meridian was measured 

 with such care and precision as to inspire confidence in the result. 

 This was done by Picard, of the French Academy. He was the first 

 to apply the telescope to instruments for the measure of angles. He 

 commenced his operations at Malvoisin, near Paris, and terminated 

 them at Sourdon, near Amiens. For the measurement of terrestrial 

 angles he used a quadrant of 38 inches radius, and for zenith distances 

 a sextant of 10 feet radius. 



He found for a degree 60.812 fathoms, a result very near the truth, 

 but it was owing to a fortunate balancing of errors which were sub- 

 sequently discovered in his work. 



5. Up to 16*72 no doubt was entertained of the perfect sphericity of 

 the earth During this year a circumstance occurred which had a 

 most important bearing upon this problem. Richer was sent out by 

 the^ French Academy to Cayenne, in South America, to make obser- 

 vations on the parallax of Mars. He took with him a pendulum, 

 carefully adjusted to beat seconds in Paris. On suspending it at 

 Cayenne, three or four degrees north of the equator, he found that it 

 lost two minutes and twenty-eight seconds per day. He shortened it 



