Chase.] -^^QQ [April. 



. . . the north end of the magnet then begins to move westward, 

 and reaches its western elongation about a quarter after 1 o'clock in 

 the afternoon (a few minutes earlier in summer). . . . The 

 diurnal curve presents but a single wave, slightly interrupted by a 

 deviation occurring during the hours near midnight (from about 10 

 p. M. to 1 A. M.), when the magnet has a direct or westerly motion; 

 shortly after 1 A. M., the magnet again assumes a retrograde motion, 

 and completes the cycle by arriving at its eastern elongation shortly 

 before 8 o'clock in the morning. This nocturnal deflection is well- 

 marked in winter, vanishes in the summer months, and is hardly 

 perceptible in the annual curve. According to the investigations of 

 Gen. Sabine, it is probable that, if we had the means of entirely 

 obliterating the effect of disturbances, this small oscillation would 

 almost disappear. In summer, when it has no existence, the magnet 

 remains nearly stationary between the hours of 8 P. M. and 3 A. M., 

 a feature which is also shown by the annual type-curve." [Ibid. p. 

 20. Comp. Hobarton Obs., 2, vi; St. Hel. Obs., 2, cxi, cxix, cxx; 

 Toronto Obs., 1, xiv, 2, xvi.] 



VI. " The critical hours which vary least during the year, are 

 those of the western elongation, and those of the morning mean de- 

 clination. The extreme diflference between the value for any month 

 and the mean annual value, is 31 minutes in the former, and 28 

 minutes in the latter." [Ibid. p. 21.] 



VII. The curves of lunar-diurnal variation "show two east and 

 two west deflections in a lunar day," the westerly maxima " occur- 

 ring about the upper and lower culminations," and the easterly 

 maxima "at the intermediate six hours. The total range hardly 

 reaches 0'.5. These results agree generally with those obtained for 

 Toronto and Prague." [Part III, p. 8. Comp. St Hel. Obs., 2, 

 xxiii, Ixxxii, cxliv; Toronto Obs., 3, Ixxxv.] 



VIII. In comparing the easterly and westerly curves, "the con- 

 stant in Bessel's formula comes out zero, and hence it is inferred 

 that the moon has no specific action in deflecting the magnet by a 

 constant quantity." [Ibid. p. 10.] 



IX. " If we take the four phases into account, the lunar action 

 seems to be retarded ten minutes, which quantity may be termed the 

 lunar-magnetic interval for the Philadelphia station. At Toronto, 

 the intervals are not so regular." [Ibid. p. 11.] 



X. " The characteristic feature of the annual inequality in the 

 lunar-diurnal variation, is a much smaller amplitude in winter than 

 in summer. Kreil, indeed, inferred from the ten-year series of the 



