APPENDIX. 



ooooo 

 No. I. 



^n Account of the Variation of the Magnetic Needle, in the 

 Eafiern States, Chap. i. p. 22. 



IN laying out lands in America, the dirrflion of the lines, is gen- 

 erally taken b\ the Ma(/i>etic Needle, Tiie inllrumenis which have been 

 generally ufcd, are the Piain Table, or the Circuinfei enter, divided into 

 degrees, and fiued wiih a Magnetic Ncedie ol ihiee or four inches radius. 

 Had the greatpfl pofTible care been laken by able mathematicians, it would 

 not have been poflible for thtm , wiih (uch iiillrumcni.s, to have avoided 

 inuny errors and millakcs, But in tcaice'lyany inflance has the variation 

 o' the needle been known, or at ail .iiicnded to. Many, and aimoif end- 

 lefs controverfies and lawfuits, have ari'en Irom this cau'e. In many in- 

 flances no data couid be found, by wh'ch it was poflible to come to a juft 

 decifion ; the variaUon of the Magnetic Netdie. at the times wlitn iliecon- 

 teRed lines were run, being unknown. On fuch accounts, the knowledge 

 of the Magnetic variations in the inland pans of ^ menca, is become a mat- 

 ter of great importance to the people ; their intereft and property in many 

 caff 5, being much afl'tted by it. 



From the year 1302, the dir(£five power of the Magnet has been em- 

 ployed with gteat fuccefs, in the aff<jirs of navig'ation. But the fit fl account 

 that we have of anv ob(erved variation in its diietlion, was by Columbus, 

 in the year 1492, in his firft voyage to Ameiica. Until tliat time, philo- 

 fophers unitoimly believed that the pole of the Magnet, exadlly coincided 

 ■with the pole of the earth ; and they had no idea of any fuch thing, as a 

 variation. Amidfl the uncommon (cenes of difficulty which oppoicd the 

 views, and fxeicifed the genius of '.he difcoverer of America, when he had 

 advanced two hundred leagues well of the Canary Ifldnds, his compaTs be- 

 gan to fail him ; and it was found not to point to the pole of the earth, or 

 exaftly north, but one degree to the weft of that point. From that tidje 

 the variatvin began to be oberved, and became more and more known. 

 For the laft century and a halt, mathematicians have made it a regular part 

 of their bufinefsto obferve it, in different parts of the earth ; with lhea«- 

 nual alterations that are conftantly taking place. 



In the year 17231 a very accurate obfervcr, G. Graham, of London, dif. 

 covered that the magnetic needle had a diurnal, as well as an annual varia- 

 tion. And it is now well known to philofophers, that from about eight 

 o'clock in the morning, the Magnetic Needle verges to the Weft, uracil a- 

 bout two o'clock in the aiternoon. When it has attained its greateft weft- 

 ward variation, it gradually returns to the eaft, until about eight or nine 

 o'clock in the evening ; when it becomes ftationary, until the next morn- 

 ing. Tables expreflive of this diurnal variation are become common ; and 

 are to be lound in the tranfaftions of all philofophical focieties. Thus in 

 the mofl regular flate of the Magnetic Needle, it is couiianiiy fubjeft to 

 two variations ; an annual z\\6. a diurnal one. 



The effeft of thefe variations are at all times fuch, that the Magnetic 

 Needle can never give to the furveyor who follov;s its di!e£fions,a firaight 

 or an accurate line. And it ought not to be ufed at all, where the bufinefs 

 requires great accuracy and precifion. It is however fcarcely prafticable in 

 America, to fubllitutc any thing better in the room of it : Moil of the lines 

 which have been already run by furveyor^; were run by the Needle ; this 



