in different Latitudes. 307 



Ancles, and that of the peaks which rise above that stratum 

 of vapours ; but the iigure of it is so uniform that pilots 

 fail into mistakes. They often remain twelve or fifteen days 

 without seeing the sun or the stars, and during that interval 

 they come to anchor, being afraid of overshooting their port : 

 but if we suppose that the inclination of the magnetic 

 neeciie in the ports to the south of Lima is known, for 

 example at Chancay, Huaura, and Santa, the dipping needle 

 will show whether it be, in regard to Lima, to the south' or 

 the north. It will sh»w at the same time opposite what 

 point of the coast a vessel is ; and this indication v;ill be 

 attended with ;nore exactness than one could hope for, be- 

 cause in these seas the inclination varies with extraordinary 

 rapidity. M. Humboldt, to whom we are indebted for 

 these remarksj observed in these seas the following values : 



Places South I-atitudes. Inclinations. 



Huancey . - 10'' 4' - 6,80'^ 



Hnaura - 11 3 - 9,00 



Chancay ,- 1 1 33 - 10,35 



T'vjse observations prove that the error of three or four 

 degrees in the inchnation in these seas would produce but a 

 idegree of error in latitude; and, on account of the tranquil- 

 lity of the Pacific Ocean, the inclination may be observed 

 to within a degree nearly- Frequent instances of such re- 

 sults may be seen in books of voyages. In like manner, if 

 on J knew exactly the inclination at the mouth of the Rio de 

 la. Plata, it would be very useful to navigators, who, when 

 the Pamperos blow, remain fifteen or eighteen days without 

 seeino- the heavenly bodies, and go on different tacks for 

 tear of losing the parallel of the mouth oi that river. 



In a word, the inclination may indicate also the longi- , 

 tude in these seas : and this m.ethod may be employed when 

 others fail. A vessel which saiUj there in the direction of 

 a parallel could not find its longitude either by a chrono- 

 meter or the declination of Haliey, unless a star could be 

 seen in order to take an horarv angle or the magnetic azi- 

 muth. The dipping needle, then, throws light on the lon- 

 gitude amidst the thickest fogs. We point out this method 

 as one of those which have only a local application ; but 

 hitherto little attention has been paid to it. These ideas 

 may be extended and rectified by able navigators. 



In general, if the inclination of the needle, and the law 

 we have tried to estabhsh, could be depended on, to observe 

 the inclination and the terrestrial latitude would be suffi- 

 cient to determine also the longitude : l)ut we have not yet 

 .examined the extent of the errors of which this methd^ may 

 U 2 be 



