12 



of the rise and fall, and the latter the horizontal movement of the 

 same fluid ; the former is governed by the curves of equal density, 

 the latter by the meridional direction of the currents. 



That there is a daily oscillation of the needle has been placed 

 beyond a doubt by observations made with the most accurate 

 instruments in almost every part of the world : the mean daily 

 change amounts to about ten minutes. When the diurnal vari- 

 ation of the needle was first discovered, it was supposed to have 

 only two changes in its movements during the day. About 7 a.m. 

 its north end began to deviate to the west, and about 2 p.m. it 

 reached its maximum westerly deviation. It then returned to the 

 eastward to its first position, and remained stationary till it again 

 resumed its westerly course in the following morning. 



When magnetic observations became more accurate, it was 

 found that the diurnal movement of the needle commences much 

 earlier than 7 a.m.; but its motion is to the east. At half-past 

 7 a.m. in England it reaches its greatest easterly deviation, and 

 then begins its movement to the west till 2 p.m. It then returns 

 to the eastward till the evening, when it has again a slight westerly 

 motion ; and in the course of the night, or early in the morning, it 

 reaches the point from which it set out twenty-four hours before*. 



Within the tropics the variations in the height of the mercury 

 in the barometer are very uniform, subsiding about half an 

 inch during the day, and rising again to its former height in the 

 night. In the northern regions, such as Denmark, Iceland and 

 Greenland, the diurnal variations are greater and less regular, in 

 the height of the mercury as well as the direction and dip of the 

 needle ; but in advancing from the north to the equator the di- 

 urnal variations diminish. In the southern hemisphere the daily 

 variation of the needle is in an opposite direction, the north end 

 of the needle moving to the east at the same hours that it does 

 to the west in the northern hemisphere. 



To give an ocular illustration, we may conceive the magnetic 

 current as a string from pole to pole : if the string be drawn 

 from its meridional position at the equator, its relative direc- 

 tion or the angle formed between it and the meridian will be the 

 same ; but if we compare the direction on each side by look- 



* See Brewster's Treatise on Magnetism. 



