128 TERRESTEIAL MAGNETISM. 



agitation of the needle. Quite differently from what is the 

 case during what I have termed magnetic storms, the needle 

 in this small westerly night excursion travels quietly from 

 one scale division to another, just as it does in the well- 

 assured and strongly characterised day movement between 

 20h. and If h. It is very noticeable, and well deserving 

 of attention, that when the needle exchanges its continuous 

 westerly movement for an easterly one, or vice-versd, it 

 does not remain for a time without altering its direction, 

 but (especially in the case of the 20 J h. to If h. period) 

 turns suddenly back. The small westerly movement 

 usually takes place between midnight and early morning; 

 but it has been noticed in Berlin, in the Freiberg subterra- 

 nean observations, at Greenwich, at Makerstoun in Scotland, 

 at Washington, and at Toronto, as early as between 10 and 

 11, or 11 and 12 hours. 



The four movements of the declination -needle recognised 

 by me in 1805( 153 ) are presented as the result of many 

 thousand two-hourly observations made at Greenwich in the 

 years 1845, 1846, and 1847, four turning hours being 

 there assigned as follows( 154 ) : Principal minimum of 

 westerly declination at 20 h. ; principal maximum at 2 h. ; 

 secondary minimum at 12h. or 14 h.; secondary maximum 

 at 14 h. or 16 h. (the declination being westerly). I must 

 here content myself with giving the mean hours, and calling 

 attention to the circumstance that in our northern zone the 

 hour of the principal morning minimum (20 h.) is not at 

 all altered by the earlier or later time of sunrise. During 

 two solstitial and three equinoctial periods, for each of 

 which Oltmanns and myself followed the march of the 

 horary variation of the declination for five or six days and as 



