480 



I.XI'EDITION TO POINT BARROW, ALASKA. 



The most pronounced feature of the diurnal variation is the morning extreme easterly deflection 

 between 7 and 8 a. m. This is in perfeet accord with the times of eastern elongation at stations in 

 lower latitudes: thus at Sitka,* 8 h a. m.; at Madison, Wis., 8J; at Toronto, 7|; at Philadelphia, 

 ?2 ; and at Key West, 8. The afternoon westerly deflection, however, appears to be delayed when 

 compared with stations to the south of Uglaamie. We have a maximum .about 5 p. m., and a 

 second and greater maximum about midnight, undoubtedly produced by disturbances, as shown 

 in the accompanying diagram. At Sitka the westerly elongation occurs about 34 p. m. ; at Madison, 

 1; at Toronto, Of p. m.; at Philadelphia, 1J; and at Key West, If. At Sitka there is no trace 

 of the irregular western deflections recorded at Uglaamie between 8 b p. m. and about 2 h a. m., as 

 shown by the table in the foot-note. If we now refer to the observations made at Point Barrow 

 during 1852, 1853, and 1854 (Phil. Trans., vol. 147, 1857), we find 8 a. m. to be distinctly the hour 

 of the maximum of the easterly disturbances, which thus re-enforce the regular solar-diurnal varia- 

 tion about this time and produce the great easterly deviation exhibited by the diagram. On the 

 other hand, the westerly disturbances reach their maximum between the hours 11 p. m. midnight 

 and 1 a. in., when they obliterate the regular solar-diurnal variation. Retaining the disturbances, 

 the eastern maximum deflection is recorded between 7 and 8 a. m.; excluding the larger ones, it 

 occurs near 7 a. m; the western maximum, disturbances included, is recorded at 5 p. in. (with a 

 second maximum between 10 and 11 p. m.), but excluding the larger ones, the elongation reverts 

 to 1 p. m. 



It is also a noteworthy fact that the diurnal variations seem to depend little on the season, 

 the deviations from the annual course for the half year with sun north of the equator, and for the 

 half year with sun south of the equator, being small. 



SEPARATION OF THE LARGER MAGNETIC VARIATIONS, OR SO-CALLED DISTURBANCES, AND 



THEIR DISCUSSION. 



In the present state of our knowledge there appears to be' no other means of recognizing so- 

 called disturbances in a series of observations except by their magnitude ; that is, for any one 

 observation or reading taken at random it is impossible to say how much of the measured quantity 

 is due to the regular daily variation, and how much to other variations following different laws. 

 Having formed preliminarily for any one month hourly average or normal values, and compared each 

 observation at any hour with the normal value at that hour, the series of differences so obtained 

 will disclose the amount of the so-called disturbances ; and a certain limiting value requires to be 

 found which shall separate the apparently regular values from the supposed disturbed values ; 

 i. e., those following different laws from the others. 



In the discussion of that large body of magnetic material which had accumulated mainly 

 through the support of the British Government about the middle of the present century, General 

 Sir Edward Sabine was guided in his selection of a limiting value simply by practical considerations 

 or by experience, and the eminent success which he had full}' justified his method; yet when a 



"It is ranch to be regretted that the magnetic observations taken at Sitka, Alaska, between 1848 and 1864, have 

 never been fully discussed. As it appeared to mo highly desirable to compare the diurnal variation of thu declina- 

 tion at Uglaamie with that of Sitka, I have made a combination of the hourly readiugs from the broken and irreg- 

 ular series extending from 1848 to 1862. (The material for this combination had been collected by Mr. M. Baker, of 

 the Coast and Geodetic Survey, in March, 1882.) 



Diurnal variation (inclusive of dinturbaiic(s) of the declination obserred at SitJca, Alaska, from ten years of observations. 

 [A + aign indicates deflection of north end of needle to the west; a sign the opposite direction. 1 



