112 ANNUAL OF SCIENTIFIC DISCOVERY. 



transition of the lines of terrestrial force there, a greater number of 

 them will be determined through the western region ; and even though 

 the lines of force may be doubted by some as having a separate exist- 

 ence such as that above assumed, still no error as to the effects on 

 magnetic needles would in that case be introduced, for they by experi- 

 ment would be and are the same. 



" The power of a magnetic body, as iron or oxygen, to favor the trans- 

 mission of lines of force through it more than other bodies not mag- 

 netic, may be expressed by the term conduction. Different bodies, as 

 iron, nickel, oxygen, conduct in various degrees ; and not only that, but 

 the same body, as iron or oxygen, conducts in different degrees at dif- 

 ferent temperatures. When space, traversed by uniform lines of mag- 

 netic force, is occupied by an uniform body, as air, the disposition of 

 the lines is not altered ; but if a better conducting substance than the 

 air is introduced, so as to occupy part of the space, the lines are con- 

 centrated in it ; or if a worse conducting substance is introduced, the 

 lines arc opened out. In both cases the lines of force are inflected, 

 and a small magnetic needle, standing in them at the inflected part, 

 would have its direction changed accordingly. Now this, by the 

 hypothesis, is assumed to take place in the atmosphere. Supposing it 

 all at mean temperature, the lines of force would have the direction 

 determined by the arrangement of the power within the earth. Then 

 the sun's presence in the east would make all the atmosphere in that 

 region a worse conductor, and as the sun came up to and passed over 

 the meridian and away to the west, the atmosphere under his influence 

 would bring up changes in that direction ; it would therefore mani- 

 festly set a needle in a given latitude in opposite directions as it passed 

 by ; and as evidently set two needles in north and south latitudes in 

 opposite directions at the same moment of time. As the night came 

 on, and a temperature lower than the mean came up from the east and 

 passed over, the lines of force would be inflected, and a reverse varia- 

 tion of the needle to that which occurred before would now take place. 

 That natural effects of variation must be produced, consequent upon the 

 magnetic nature of oxygen and its daily variations of temperature, is 

 manifest ; but whether they cause the observed variations, or are com- 

 petent to do so, is a question that can only be decided after a very 

 careful inquiry. Observations are now made on the surface of the 

 earth with extreme care in many places, and these are collated, and 

 the average or mean result, as to direction and intensity of the earth's 

 force, ascertained for every hour and season ; and also many remarka- 

 ble anomalous and extra results evolved. A theory of the causes of 

 any or all of these variations may be examined, first by the direction 

 which the varying needle does or ought to assume, and then by the 

 amount of the variation. The hypothesis noAV brought forward has 

 been compared with the mean daily variation for all the months in the 

 year, at north and south stations, as Toronto and Hobarton, and at 

 many others near to and far from the equator, and agrees in direction 

 with the results observed far beyond what the author anticipated. 

 Thus the paths described by the upper ends of free needles in the north 

 and south hemispheres, should be closed curves, with the motion in 



