Gravity on Magnetic Declination. 187 



V. "The general character of the diurnal motion is 



nearly the same throughout the year ; the most eastern deflec- 

 tion is reached a quarter before 8 o'clock in the morning (about 

 a quarter of an hour earlier in summer, and half an hour later 



in winter) ; the north end of the magnet then begins to 



move westward, and reaches its western elongation about a 

 quarter after one 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 percep- 

 tible in the annual curve. According to the investigations of 

 General Sabine, it is probable that if we had the means of en- 

 tirely 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. 

 Helena 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 

 declination. The extreme difference 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 



• " occurring 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. 

 Compare St. Helena Obs. 2. xxiii. lxxxii. cxliv. Toronto Obs. 

 3. lxxxv.). 



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

 constant in Bessel's formula comes out zero, and hence it is in- 

 ferred 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 



