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needle, which is by no means the case: the eastern line of no va- 

 riation is much more irregular, being full of loops and inflexions: 

 it begins in latitude 60° south, below New Holland, crosses 

 that island through its centre, and extends through the Indian 

 Archipelago with double sinuosity, so as to cross the equator 

 three times : it then stretches along the coast of China, making 

 a semicircular sweep to the west till it reaches the latitude of 

 71°, where it descends again to the south, and returns north- 

 wards with a great semicircular bend, which terminates in the 

 White Sea. In the Pacific Ocean we find elongated ovals de- 

 scribed ; what the centre of such ovals intends to represent it is 

 difficult to imagine. Such is the confused manner in which the 

 direction and variation of the magnetic needle are represented. 

 If the chart were compared with Plates IV. and V., on which the 

 same magnetic meridians are indicated by the arrows, it would 

 scarcely be believed that they represented the same phenomena. 

 It is to be hoped, for the sake of mariners as well as for the pro- 

 gress of science, that the lines will be shown in their simple 

 natural state, and not confuse the phenomena with unmeaning 

 lines, to suit formulae which can never be of any utility. 



Rocks. 

 We have shown how essential is the knowledge of the prin- 

 ciple, not only for surveying lands, but for studying the structure 

 of rocks, their modifications, disturbances and general character. 

 By knowing the nature of the formation, the growth as it were 

 of a series of crystalline rocks, their grain, and the laws of action 

 by which they are controlled, we are enabled, first, to make a 

 systematic examination, and the results must be more definite 

 and satisfactory than it possibly can be without any rule or 

 order. In the proper selection of building materials, it will teach 

 us that rocks of the same nature are very different in structure, 

 composition and durability, according to their geological position 

 and the variable state of the compound. Like timber, so it is 

 with stone; it is not equally durable in all situations, nor equally 

 compact in situ. 



To the mine captain this grand law of nature has been of im- 

 portant service in carrying on his subterranean operations, but 



