332 Astronomy. [Lecture 21. 



1635, took the sun's altitude when it was in the 

 summer solstice, both at London and York, with 

 a sextant of five feet radius, and by that means 

 found the difference of latitude between these two 

 cities to be two degrees and twenty-eight minutes. 

 He then measured their distance in as exact a man- 

 ner as he was able ; and having taken into the 

 account all the windings of the road, with the as- 

 cents and descents, he reduced it to an arc of the 

 meridian, and found it to contain twelve thousand 

 eight hundred and forty-nine chains ; and this 

 distance, being compared with the difference of 

 latitude, gave him five thousand two hundred 

 and nine chains to a degree, or about fifty-seven 

 thousand three hundred French fathoms or toises. 

 This method requires no explanation, if the 

 two places are considered as lying under the 

 same meridian, which indeed is nearly the case. 

 The same operation may also be easily performed 

 by trigonometry, when the two places lie under 

 different meridians; for if we measure the di- 

 stance of any two objects and take the angles 

 which each of them makes with a third, the tri- 

 angle formed by the three objects will become 

 known ; so that the two sides may be as accurate- 

 ly determined by calculation, as if they had been 

 actually measured in the same manner as the first. 

 And by making either of these sides the base of 

 a new triangle, the distances of other objects may 

 be found by trigonometry as before ; and thus, 

 by a series of triangles connected together at their 



