1377.] \)4:u [Frazer. 



6th. The situation of this curve cannot be very near tlie Terrestrial Pole, 

 since on the same parallel of latitude and with a distance apart of about 

 2,685 miles, Passamaquoddy Bay shows a (West) declination of + 18° and 

 Salem, Oregon, an Easterly declination of — 20° for the same year. These 

 curves unless forming abnormally acute cusps must meet if produced be- 

 low lat. 70° as drawn in Col. Sabine's chart. 



7th. It is probable that the location of the area of magnetic attraction is 

 nearer to the Atlantic than to the Pacific, because the Isogonic curves of 

 even degrees are nearer together in the former region than in the latter. 



8th. Any theory which may be established to account for secular varia- 

 tion must accept the end of the last century or the beginning of this as one 

 of the extremes. 



9th. The total period of revolution according to the best data is about 

 237* years with a margin of a few years error more or less. 



10th. The limits within which the declination for a particular date may 

 be calculated for a station, where reliable observations are at hand, is about 

 8' to 11'. 



The following table was calculated to represent the magnetic variation 

 at Mount Holly, Cumberland Co., Penna., for every five years since 1790, 

 and may serve as an example of how such approximately accurate tables 

 may be obtained for points where the records are very meagre. 



In the first place the Isogonic chart of the Coast Survey forl870 (as cor- 

 rected by the pen of Mr. Chas. A. Schott) was referred to for the positions 

 of various isogonic curves which passed at different dates in the vicinity of 

 Mount Holly. The exceedingly small scale of this chart and the uncer- 

 tainty of the data from which many of the curves were traced, renders the 

 attainment of the required declination only possible within rather wide 

 limits, amounting perhaps to 20' if all errors be taken into account ; or a 

 little more than the minimum (quarter of a degree) noticed by an ordinary 

 surveyor. 



Of course, if the chart be assumed as correct, the error is reduced to less 

 than half this. The method employed for determining these declinations 

 is as follows : After locating the position of the place whose declination is 

 sought on the chart by latitude and longitude, a normal to the isogonic 

 curves of the date required is drawn through this place. The distance 

 apart on this line of the nearest curves of whole degrees between which the 

 place lies, is accurately measured, and afterwards the distance from the 

 point to that one of these curves which is represented by the smallest num- 

 ber of degrees. This distance in the same units multiplied by sixty and 

 divided by the former number will give the number of minutes, which be- 

 ing added to the smallest number of degrees represented by the two curves, 

 furnishes the correct declination of the place at the date for which the curves 

 are drawn. If any actual observations of the declination at the desired lo- 

 cality are at hand these may be compared with the nearest place at which 

 observations have been carried on, and the annual rate of change of the 



* See Mr. Schott's Report, atite. 



