498 Geology and Magnetism. 



appear to be the effects of the oscillating movements of the magne- 

 tic currents. 



The mercury in the barometer, like the magnetic needle, fluc- 

 tuates without being accompanied with a visible aurora. Probably 

 the light is produced by a change in the constituent elements; 

 but the light is not essential to prove the existence of the currents ; 

 nor is visibility necessary to produce undulations in such currents : 

 such variations can only be ascertained by their effects. 



That the barometer is subject to perpetual oscillation is a pheno- 

 menon so well known as not to require comment ; but that the 

 magnetic needle should be also subject to similar oscillations ap- 

 pears to those who have not paid attention to the subject somewhat 

 strange. However, the fact is, that the former is the effect 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 in- 

 struments in almost every part of the world : the mean daily change 

 amounts to about ten minutes. When the diurnal variation 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 east- 

 ward 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 

 * See Brewster's Treatise on Magnetism. 



