Sciences of Meteorology and Terrestrial Magnetism. 183 



that producing dip, or inclination. The inclination is properly the 

 angular amount of dip. Now these have beeu separated, and estimated 

 as the horizontal and vertical components of the force. An admirable 

 contrivance for the purpose is due to the ingenuity of Dr. H. Lloyd of 

 Dublin. The total force is estimated iu numbers, the unit being 1 grain 

 in weight, 1" in time, and 1 foot in space; or the force is such as in 1" 

 would generate in 1 grain a velocity of 1 foot — ^just as the force of gravity 

 is 32|^, though in the first second 16^ feet is the space passed over. 

 Estimated thus, the force at the point of maximum intensity in Canada 

 is 14-21 ; at Toronto, 13-896 ; at Greenwich, 10-388. In the southern 

 hemisphere, it is 15-600 — the point of maximum intensity there best 

 ascertained being about lat. 60° S., long. 135" E. Another intensity 

 pole is in lat. 20° S., Ion. 36° W. These four foci or intensity points 

 aj-e constantly changing their positions — the two northern shifting 

 eastwards, and the two southern westwards ; the weaker, or eastern 

 pole, in Eastern Siberia, moving much faster than the stronger or 

 western, in Canada ; so that they are approaching one another in a 

 line, crossing from Siberia, through Russian America, towards the south 

 of Hudson's Bay. In this intermediate space the total force is increas- 

 ing. — To other determinations and results I cannot now allude, and 

 shall further only state a few facts, very recently ascertained, bearing 

 closely on the theory of terrestrial magnetism, and tending to with- 

 draw the science from the category of terrestrial agencies, and to place 

 it in the class of the " great cosmical phenomena." 



(26.) A liorizontal magnet has variation with the hours of the day, so 

 that, running through several changes in the advance of the hours, from 

 sunrise to sunrise, it retui-ns to its former position at the expiry of the 

 time, to begin a new set of variations. It attains its maximum when 

 the sun is 2'' past the meridian of the place of observation, — say any- 

 where in Europe. — With us this would be 2'' p.m. ; but at Constanti- 

 nople it would be 4 p.m. of our time ; and at the Azores lO"" a.m. of 

 our time. This change, then, is clearly dependent on the sun's passage 

 of the meridian, of IIm observer. Now, this diurnal variation is not the 

 same at all times of the year, — it runs through a series of changes, the 

 period of which is one year ; so that the diurtial variation has an anniial 

 period ; the same condition of things beuig again established on the 

 expiry of the year, and coming round again the next year in like order. 

 But the variations are not the same in each of the half-years forming 

 the annual period. They difter with the lapse of the two semi-annual 

 periods from April to September, and from October to March. These 

 changes, however, have no relation to summer and winter, or to the 

 Beacons of the j-^ear ; for they correspond most remarkal)ly at the three 



