402 



rect knowledge of the regular periodical variations, it would be found 

 necessary to eliminate the " casual perturbations," has been fully 

 confirmed. Had the latter been strictly "casual" (or accidental, in 

 a sense contradistinguished from and opposed to periodical), a suffi- 

 ciently extended continuance of observation might have occasioned 

 their mutual compensation ; but now that we have learned that the 

 mean effects which they produce are governed by periodical laws, and 

 that these laws and those of the regular periodical variations are dis- 

 similar in their epochs, it is manifest that in their joint and undivided 

 effects we have two variations, due to different causes and having di- 

 stinct laws, superimposed upon each other; to know the one correctly 

 we must necessarily therefore eliminate the other. A striking illus- 

 tration of the importance of such elimination is furnished by the solar- 

 diurnal variation of the total force. It will readily be imagined that 

 the question must be an important one, whether a variation, which 

 is supposed to derive its origin from the sun, be a single or a double 

 progression ; whether it have two maxima and two minima in the 

 twenty-four hours, or but one maximum and one minimum in that 

 period. When no separation is made of the disturbances, the pro- 

 gression appears to be a double one, having two minima, one occur- 

 ring in the day and the other in the night. With the removal of 

 the disturbed observations the night minimum disappears, and we 

 learn that the regular solar-diurnal variation of the total force has 

 but one notable inflection in the twenty-four hours, viz. that which 

 takes place during the hours when the sun is above the horizon. 

 The night minimum is in fact the mean effect of the occasional dis- 

 turbances. It is probable that the nocturnal inflection of the solar- 

 diurnal variation of the Declination may be ascribed to the same 

 cause, namely to the superposition of two distinct variations. 



A careful analysis of the solar-diurnal variations of the Declination 

 at the Colonial Observatories has brought to light the existence 

 at all these stations, of an annual inequality in the direction of the 

 needle concurrent with changes in the sun's decimation, having 

 its maxima (in opposite directions) when the sun is in or near the 

 opposite solstices, and disappearing at or near the epochs of the 

 equinoxes. An intercomparison of the results of the analysis at 

 these stations has shown, that this inequality has the remarkable 

 characteristics of having notably the same direction and amount in 



