394 



M. A. Secchi on the Periodical Variations 



hours before the sun passes the meridian of the place; it then turns 

 eastward with increasing celerity, of which the maximum occurs 

 near the passage of the sun through the magnetic meridian, and 

 it reaches its limit of eastern excursion one or two hours after 

 the said passage. As the sun declines, the needle returns back ; 

 and as the sun passes the inferior meridian, there is repeated in 

 the night the same variation as that which took place during the 

 day, but restricted within narrower limits. The limiting hours 

 of these changes vary with the seasons, and are generally earlier 

 in summer and later in winter ; and the magnitudes of the ex- 

 cursions are in the proportion of the diurnal to the nocturnal 

 arc/' 



Declaration. — The wood-cut will make this law better under- 

 stood. Let EW be the equator or the parallel described by the 



sun, and T, H, two places situated in opposite hemispheres as 

 respects the sun; between the hours of 19^ and 21^ (7 and 

 9 A.M.) the two needles will be in the positions shown on the 

 line SN; from 1 to 2 p.m., in the position shown on the line 

 S'N'; between 9 and 10 p.m., as shown on the line S"N"; and, 

 finally, between 14^ and 15^ (2 and 3 a.m.), as on the line 



The case here represented is exactly that of Toronto (T), and 

 Hobarton (H). Colonel Sabine describes the diurnal oscillation 

 at Toronto as follows: — ^*The mean diurnal variation of the 

 declination at Toronto, as derived from the two-hourly observa- 

 tions in 1841 and 1842, consists in an easterly movement of the 

 north end of the magnet from 2 to 10 hours inclusive ; a small 

 return movement towards the west then takes place till 14 hours, 

 when the easterly progression is resumed, and continues until 

 20 hours, at which time the north end of the magnet reaches its 

 eastern limit. From 20 hours the movement is continuous towards 



