ON ASTRONOMY. 



93 



TugS 



y A 



In 1750 Boscovicli and Le Maire measured an arc from Eome to 

 Rimini, a distance of nearly 140 miles. 



In 1752 Liesganig measured an arc passing througli the observatory 

 of Vienna, and about the same time Beccaria measured another in the 

 plains of Lombardy. 



In 1764 Charles Mason and Jeremiah Dixon measured a line of 

 about 100 miles in length along the valley of the Delaware. They were 

 engaged at the time in running the boundary line between the States of 

 Pennsylvania and Maryland, and finding a finelocality for measuring 

 an arc of the meridian, they informed the Royal Society of London 

 of the fact and offered to execute the work if the Society would iurnish 

 the requisite instruments. The Society, under the advice of Dr. Mas- 

 kelyne, the astronomer royal, gladly accepted the offer of Messrs. 

 Mason and Dixon. This line is re- 

 markable as having been actually 

 measured through its whole extent 

 with wooden rods,or rather a wooden 

 frame, sent out for the purpose by 

 the Royal Society. It was made of 

 fir, 20 feet long and 4 feet wide, with 

 adjustments for being leveled by 

 plumb line. This measurement has 

 always been regarded as a very ac- 

 curate one. The latitude of the 

 middle point of the line is 39° 12' 

 nearly. This arc is one of the 13 

 used by Airy in determining the 

 figure of the earth. 



10. After the peace 1783 Cassini de Thury addressed a memoir to the 

 Royal Society of London upon the importance to astronomy of deter- 

 mining more accurately than had before been done the difference in 

 longitude between the observatories of Paris and Greenwich. The 

 suggestion was favorably received and commissions were appointed by 

 both governments to carry the project into effect by a trigonometrical 

 survey. General Roy was at the head of the English commission.^ 

 The French operations were committed to Count Cassini, (the 4th of 

 the name,) Mechain and Legendre. 



The English had hitherto taken little part in the prosecution of 

 these important geodetic labors, but the splendid array of instru- 

 ments and the practical skill which they brought to the work placed 

 them second to no others. The great theodolite, constructed for the 

 purpose by Ramsden, 30 inches in diameter, may almost be said to be 

 the wonder of the age. It certainly marks an important epocli in the 

 history of astronomical instruments. The three angles of every spheri- 

 cal triangle are well known to be greater than two right angles. But 

 this theodolite, in the hands of General Roy, was the first instrument, 

 in the history of science, to show the fact by actual measurement. It 

 is recorded, as a singularly gratifying fact, that in all the primary tri- 

 angles this spherical excess was apparent, and seldom did it exceed 

 3" or 4" of arc. 



The work on both sides of the channel was executed with eminent 



