432 M. A. Secchi on the Periodical Variations 



object is EQ. In the time that light takes to pass from p to E, 

 let the object move from p to P, so that EP is its real direction 

 at the instant of observation. Consequently the Z.QEP is the 

 difference between the instrumental and the real directions. 

 Draw Q<? parallel and equal to pY, and complete the parallelogram 

 QqYp. Now the earth being conceived to be at rest, the ap- 

 parent angular motion of the body in the interval of the transit 

 of light.from p to E is the angle subtended at E by the resultant 

 QP of the motion Q^ of the body and the motion Qp equal and 

 opposite to that of the earth, viz. the Z QEP. Hence EQ, the 

 instrumental direction of the body at the time of observation, 

 coincides with its actual direction at a time preceding the obser- 

 vation by the interval occupied by the passage of the light from 

 the body to the observer. This result establishes the rule it was 

 proposed to demonstrate. 



It follows from this theory, that the instrumental direction of 

 a terrestrial object, whose motion in space is parallel and equal 

 to that of the observer, coincides with the actual direction ; and 

 that the instrumental direction of a fixed object, as the sun, and 

 of an object having a fixed direction, as a star, differs from the 

 actual direction by the whole amount of instrumental aberration. 

 The theory also shows that the value of the constant of aberra- 

 tion may be in some degree affected by the retardation which 

 the light undergoes in that part of its course between the direc- 

 tion-points and W, which lies within the substance of the 

 object-glass, or passes through other glasses. 



Cambridge Observatory, 

 May 11, 1855. 



LXI. On the Periodical Variations of Terrestrial Magnetism. 

 By A. Secchi, Director of the Obsewatory of the Collegio 

 Romano*. 



[Concluded from vol. viii. p. 399.] 



Part II. § 2. On the Variations of the other Magnetic Elements. 



THE two magnetic elements, the variations of which remain 

 to be determined, are the intensity and the inclination. 

 But as their direct determination is very difficult, an indirect 

 method is used, which consists in finding the variations of the 

 components of the total force resolved along three coordinate 

 axes taken with reference to the horizon. 



The position of the dipping-needle shows the direction of the 

 resultant of all the magnetic forces of the globe ; and the direct 

 method of determining the intensity of this force would be to 



* Communicated by Mr. Archibald Smith. 



