128 Rev. Mr Scoresby on the Uniform Permeability of 



Case ll.'^Where the direction may he certain^ hut the level douhtful. 



Here the magnet and compass being placed on the known 

 line of direction, the compass is moved progressively upward, 

 from the floor to the roof of the n)ine, to determine the greatest 

 deviation, which, it is evident, will be on the same horizontal 

 level as that of the magnet. 



A result, perhaps more satisfactory, might be obtained by the 

 use of a needle, suspended hke a dipping needle, and traversing 

 in a vertical plane. Such a needle being brought into a vertical 

 position by a small directing magnet, would shew by its devia- 

 tions the position of corresponding level. 



Case III. — Where both level and direction may he uncertain. 



In this instance, the magnet being fixed, and the plank placed 

 on the floor of the mine, the compass apparatus is to be car- 

 ried from side to side, till the place of the greatest deviation 

 is ascertained, or the middle between the two positions of great- 

 est deviation produced by the different poles. This will give 

 the directional position. The vertical needle being now tried 

 throughout a vertical line drawn through the directional point, 

 will shew by its greatest deviation the horizontal level of the 

 magnet, which, being at the same time in the directional line, 

 will indicate the nearest attainable point between the magnet 

 and compass. The vertical needle, in this and similar cases, 

 may be guided in its transit by a board, with a straight edge, 

 or pillar of wood, placed vertically, so as to preserve its paral- 

 lelism of position at every altitude. 



It may be proper here to call to mind, that the chief object 

 of the processes now described, . is the determination of the 

 thickness of solid substances not otherwise measurable, and 

 not primarily at least, the determination of direction and levels ; 

 for the application of the observation of deviations to these 

 latter purposes is only expected to afford an approximation, 

 in some cases, indeed, a very important approximation, to the 

 truth. Where the distance to be measured is small, say only 8 

 or 10 feet, or in any case in which the deviations amount to 3 

 or 4 degrees, — the line of direction may be very accurately de- 

 termined by the middle point between the maximum deviations 



