MAGNETIC OBSERVATIONS. 341 



of a thin sheet of magnetic rock lying at a low angle of dip, the buried 

 north poles would not be much farther removed than the upper south poles, 

 and consequently the compass needle should be relatively only slightly dis- 

 turbed. This is precisely what is found to be the case. 



Thus there is firm ground for the conception of the magnetic rocks as 

 made up of sheaves of small mag-nets, all similarly oriented in a general 

 way and all having their south poles upward at or near the rock surface, 

 while the effective north poles, by the continual addition of similarly ori- 

 ented sheaves below, are carried down, when the rocks are vertical or nearly 

 so, to such depths that their influence is greatly diminished or altogether 

 imperceptible. In equal small areas the individual magnets are no doubt 

 of very unequal numljer or strength. This can be proved by holding a 

 swinging needle close to the surface of a magnetic rock, shifting its position 

 without moving it out of the parallel plane and observing the changes in 

 the pointing. These are almost always large and are undoubtedly due to 

 the variations in strength of small areas of the upper poles. In consequence 

 of the law of magnetism, by which the attraction (or repulsion) varies 

 inversely as the square of the distance, the areas immediately surrounding 

 the needle are very much more important factors in the resultant than those 

 farther removed. When the needle is held higher up, or, what is the same 

 thing, the magnetic rock is buried, the effects are much more regular, since 

 a larger number of the unit areas enter into the resultant with equal weight 

 due to equal distance, and the extremes of individual variation are lost in 

 the general mean. Since successive magnetic cross sections over buried 

 rocks show on the whole a great degree of regularity, we can finally con- 

 clude that the magnetic force of these rocks is seated in an immense, prac- 

 tically an infinite, number of small magnets, which furnish free magnetism 

 at the upper and lower bounding surfaces of the magnetic formation, and 

 that on the average there is about the same number, of about the same 

 aggregate strength, or, in other words, equal intensity in equal areas of 

 these surfaces, if the areas are taken large enough. 



SECTIOlSr IV. THE INSTRUMENTS AND METHODS OF WORK. 



The instruments used in this work are simple and well known. The 

 dial compass is an ordinary compass, carrying a 2^-inch needle swinging 

 inside a circle graduated to degrees, which is further supplied with a grad- 



