Prof. W. A. Norton on Terrestrial Magnetism. 211 



as stationary for an indefinitely small change in the position of 

 the place, and determine the direction of the isothermal or isoge- 

 othermal line on this supposition, the needle makes a right angle 

 with this direction. In point of fact however, these poles change 

 their situation with every change in the position of the place ; 

 and hence, formula (6) will not give the true direction of the 

 isothermal line. But the amount of the error will be quite dif- 

 ferent in different places. In high latitudes the displacement of 

 the ideal cold poles is slight, and less in proportion as the place 

 is nearer to either of these poles; and accordingly formula (6) 

 gives very nearly the true direction of the isothermal line. From 

 the meridian of Paris westward, across the Atlantic Ocean, and 

 throughout the United States, while the change of longitude 

 ot the Asiatic pole is equal to that of the place, the change in its 

 distance from the place from this cause is slight; since, as it has 

 been seen, this pole is almost directly opposite to the place on 

 the other side of the geographical pole. The motion of the 

 American pole is much less, and tends to diminish the variation 

 of the computed temperature produced by the motion of the 

 other; since, while 8 is increased by the one, 8' is diminished by 

 the other, and thus the variation of sm n 8 shW in equa. (4). is 

 Jess than would arise from the change of either 8 or 8' alone, 

 fhroughout the portion of the earth's surface just designated, 

 therefore, formula (6) gives nearly the true direction of the iso- 

 hermal line. In passing eastwardly from the meridian of Paris, 

 oy er Europe and Asia, the displacement of the Asiatic pole is 

 JJore rapid, while that of the American pole is about the same, 

 the two tend to counteract each other, but it would seem that 

 he effect of the former must preponderate, and therefore that 

 he true direction of the isothermal line may materially differ from 

 [hat given by formula (6). The indications are, however, that 

 "e error lies in one direction over Europe and Asia, and in the 

 other direction across the Pacific Ocean, and thus, that the iso- 

 thermal line as traced by means of formula (6) would no where 

 fitter very widely from the truth. 



Un referring to a chart of isothermal lines, constructed from 

 observations, (see Kaemtz's Complete Course of Meteorology, 

 rench edition, Plate VI,) it will be seen that generally the 

 J°»jse of these lines is in fact similar to that of the lines traced 

 p formula (6. ) Thus, in crossing the continent of North Amer- 

 ca , from the western to the eastern coast, they tend south of 

 381 J and in crossing the Atlantic Ocean, from America to Europe, 

 oorth of east. From Western Europe to the eastern coast of 

 * Sla , their direction is south of east. The points of inflexion, or 

 . ekmge of direction from one side to the other of the east and 

 est line, of the two systems of lines, are however generally 

 parated by a moderate interval. The comparison is here con- 

 ned to the Temperate and Frigid Zones. 



