Vertical Component of Magnetic Force. 



379 



Diurnal Fariation of the Vertical Component. — The following is the meau result for the year 1844 : — 

 The vertical component is a maximum at 6*^ 20™ p.m., after wliich it diminishes mth considerable rapidity till 

 midnight, the principal minimum occurring about l"" 30™ a.m. ; it then increases till Q^ 10" a.m, when it is 

 a secondary maximum, becoming a secondary minimum at IC" 40™ a.m., after which it increases to the princi- 

 pal maximum at S*" p.m. The sccondai7 maximum and minimum are distinctly marked, but the variation from 

 the former to the latter is inconsiderable, being only 0-000010, while the variation from the principal ma.ximum 

 to the principal minimum is 0-000420, the whole vertical component being unity. 



The form of the diurnal variation of the vertical component varies more with season than that of the 

 diurnal variation tov either of the elements already discussed. In winter the diurnal variation is single, the mini- 

 mum occurring about S"" a.m. and the maximum about G*" p.m. In the months near the equinoxes the range 

 of the variation is greatly increased, and its form is similar to that for the year given above ; the secondary 

 maximum and minimum become gradually more marked as the epoch approaches the summer solstice, till in 

 the months of June and July the principal minimum occurs about noon, the range of the diurnal variation hav- 

 ing diminished at the same time to nearly the same value as at the winter solstice. The follo\ving are the ap- 

 proximate times of maxima and minima in each month of the year, the principal ma-ximum being indicated by + 

 and minimum by — 



The principal maximum occurs earliest at the equinoxes : the principal minimum occurs farthest from 

 midnight at the winter solstice and nearest it as we approach the summer solstice. The principal minimum 

 actually occurs near noon at the summer solstice, but the minimum near midnight differs little in value. 



Adopting the method already pursued (see page 339, for the purpose of considering the diurnal variation 

 when freed as far as possible from the effects of irregular causes, the following Tables have been formed. 



TABLE XLIV. — Hourly Means of the Balance Magnetometer Micrometer Readings for the Ten 



Days least disturbed in each Month of 1844. 



