S8 Rev. Mr Scorcsby's Ohservatums 07i the 



ship's head, indicated the quantity of local attraction on the seve- 

 ral courses. 



Another method which I adopted in my own practice was still 

 more simple. A compass was secured at the main top-gallant- 

 mast-head, where, being remote from all iron, and directly above 

 the attraction of the ship, it was found to give the correct mag- 

 netic position on every course alike. This was made the stand- 

 ard compass. Comparing, therefore, the course steered by the 

 biimacle^compass with that indicated by the standard, which 

 could be done as frequently as requisite in calm weather and 

 smooth sea, the deviation in that particular position of the ship's 

 head was at once determined. Occasionally a whole series of 

 differences was observed, so that the deviation on every course 

 might be known. 



A beautiful and philosophical detector of the deviation has for 

 some time been in use in the navy, the invention of Mr Barlow, 

 in which a plate of iron is temporaneously affixed in proximity 

 to the steering compass, so as exactly to double the influence of 

 the ordinary attraction of the ship ; this increase upon the usual 

 deviation affords, if the position of the plate be correct, a mea- 

 sure of the local attraction produced by the iron in the vessel. 

 The discovery of the position of the plate, however, is a matter 

 of more experimental nicety than can be usually expected from 

 the men of any profession as a body ; and no provision, that I 

 am aware of, short of a new determination of the position of the 

 plate, can be obtained for such accidental changes of the local 

 attraction, as ships in general are liable to on any change of po- 

 sition in their guns or other masses of iron on board. Where, 

 however, the iron in the vessel remains unaltered, both in quan- 

 tity and position, Mr Barlow's plate will, unquestionably, be 

 found capable of exhibiting the influence of the local attraction, 

 generally, throughout the globe, not only with useful, but even 

 philosophical, accuracy. 



Another invention for the same purpose remains only to be 

 noticed, which is, the beautiful apparatus of Lieutenant-Colonel 

 Graydon of the Engineers, denominated by him the " Celestial 

 Compass.*" 



This ingenious instrument, by a beautiful arrangement of 

 graduated arcs and circles, is so adjusted for the latitude of the 



