TERRESTRIAL MAGNE1ISM. 171 



simultaneous manifestation of the storm may serve, within 

 certain limitations, like Jupiter's satellites, file-signals and 

 well- observed falls of shooting stars, for the geographical 

 determination of degrees of longitude. We here recognise 

 with astonishment, that the perturbations of two small mag- 

 netic needles, even if suspended at great depths below the 

 surface, can measure the distances apart at which they are 

 placed, teaching us, for instance, how far Kasan is situated 

 east of Gottingen, or of the banks of the Seine. There are also 

 districts in the earth where the mariner, who has been en- 

 veloped for many days in mist, without seeing either the sun 

 or stars, and deprived of all means of determining the time, 

 may know with certainty, from the variations in the incli- 

 nation of the magnetic needle, whether he is at the north or 

 the south of the port he is desirous of entering.* 



portions of the Earth's surface, and -winch are collected in Sabine'a 

 important work (Obsew. on days of unusual magnetic disturbance^ 

 1843), one of the most remarkable is that of the 25th of September, 

 1841, which was observed at Toronto in Canada, at the Cape of Good 

 Hope, at Prague, and partially in Van Diemen's Land. The English 

 Sunday, on which it is deemed sinful, after midnight on Saturday, to 

 register an observation, and to follow out the great phenomena of creation 

 in their perfect development, interrupted the observations in Van 

 Diemen's Land, where, in consequence of the difference of the longitude, 

 the magnetic storm fell on the Sunday. (Observ., p. xiv. 78, 85 and 87.) 

 * I have described, in Lametherie's Journal de Physique, 1804, 

 t. lix. p. 449, -the application (alluded to in the text) of the magnetic 

 inclination to the determination of latitude along a coast running north 

 and south, and which, like that of Chili and Peru, is for a part of the 

 year enveloped in mist (garua). In the locality I have just mentioned, 

 this application is of the greater importance, because, in consequence of 

 the strong current running northwards as far as to Cape Parefta, navi- 

 gators incur a great loss of time if they approach the coast to the north 

 of the haven they are seeking. In the South Sea, from Callao de Lima 

 harbour to Truxillo, which differ from each other in latitude by 3 57', 

 I have observed a variation of the magnetic inclination amounting to 9 

 (centesimal division) ; and from Callao to Guayaquil, which differ in 

 latitude by 9 50', a variation of 23'5. (See my Relat. hist., t. iii. 

 p. 622.) At Guarmey (10 4' south lat.), Huaura (11 3' south lat.), 

 and Chancay (11 32' south lat.), the inclinations are 6'80, 9, and 10'85 

 of the centesimal division. The determination of position by means 

 of the magnetic inclination has this remarkable feature connected with 

 it, that where the ship's course cuts the isoclinal line almost perpen- 

 dicularly, it is the only one that is independent of all determination of 

 time, and consequently of observations of the sun or stars. It is only 



