OF THE MAGNETIC DECLINATION. 



DIURNAL DISTURBANCE VARIATION OP THE DECLINATION. 



19 



The curve has but one maximum and one minimum ; its most prominent feature 

 is the easterly deflection at 8 o'clock ( + 19i m ) P. M. (at Toronto it is at 9 P. M.). 

 At that hour the maximum deflection amounts to 32" of arc, and to 45" at Toronto. 

 The greatest westerly deflection occurs at 6 h ( + 19i m ) A. M., and amounts to but 

 14"; the Toronto hour is 8 A. M. with 6", and from a five years' series of observa- 

 tion, with 31" of deflection. The range of the disturbance variation equals 46". 1 

 The disturbance amplitude, as well as the regular variation amplitude, is greater 

 at Toronto than at Philadelphia, the occurrence of the maximum and minimum 

 disturbance deflection seeming to be about one hour earlier at the latter station. 

 From three in the morning till five in the afternoon the mean effect of the dis- 

 turbances is to deflect the north end of the magnet to the west, and during the 

 remaining hours (principally at night) to the east. The westerly and easterly dis- 

 turbance deflections during a day balance within 0'.02. 



The annual inequality in the amplitude of the diurnal disturbance variation 

 might be satisfactorily shown by the proper combination of the results for consecu- 

 tive years, comparing each two-year series successively ; but owing to the small 

 amount of the amplitude itself, and the incomplete or partly interrupted series of 

 observations in the years 1840, 1843, and 1845, it was thought best to restrict the 

 present discussion to the mean disturbance variation. 



It is my intention to continue the discussion of the observations made at the 

 Girard College Observatory. 



After the above was written, 2 No. 1185 of the Astronomische Nachrichten came 

 to hand, containing Prof. R. Wolf's interesting results on the close connection of 

 the variation in frequency of the solar spots, and the corresponding inequality in 

 the amplitude of the diurnal variation of the declination. He deduces for Munich 

 the formula /3 = 6'.273 -f- O'.Ool a in which a is a relative number expressive of 



1 At Toronto 51", and from a five years' series 83". 



8 For former communications by Prof. R. Wolf, see Nos. 839, 1043, 1091, 1132, 11(50, and 1181, ibid. 



