294 Analyses of Books. [Oct. 



Article IX. 



Analy.ses of Books. 



An Essay on Magnetic Attractions, particularly as respects the 

 Deviation of the Compass on Ship-Board, occasioned by the 

 local Injiaence of the Guns, &)C. With an easy practical 

 Method of observing the Same in all Parts of the World. By 

 Peter Barlow, of the Royal Military Academy. 8vo. London. 

 Taylor. 1820. 



The deviation of the compass on ship-board from the iron in 

 the vessel, and the great errors into which navigators are liable 

 to fall, in consequence of this deviation, first attracted the atten- 

 tion of Captain Fhnders, while employed in surveying the coasts 

 of New Holland. After a great deal of investigation, he hit upon 

 a method of correcting these errors in his ship. This method 

 was explained in a paper which Captain Flinders transmitted to 

 the Royal Society, and which was printed in the Transactions of 

 that learned body. After Capt. Flinders's return to this country 

 he was employed by the Admiralty to make a set of observations 

 on board different vessels in the Channel to ascertain whether 

 his method of correcting the deviation was generally accurate 

 It is well known that the result of the investigation was unfa- 

 vourable. His rule, though it seems to have held good, or very 

 nearly so, in his own vessel, was found not to apply with equal 

 facihty to other vessels and other circumstances. During the 

 late voyages of discovery to the north pole, the attention of the 

 public was again drawn to this subject, and many curious obser- 

 vations were published, by Captain Sabine, and others ; but no 

 attempt was made to discover a new method of correcting the 

 deviation. 



It seems to have been this voyage which led Mr. Barlow to 

 investigate the subject ; and he was led by a set of experiments, 

 •which he relates in the little volume now under review, or the 

 ■result of these experiments, to a practical method of correcting 

 the deviation, or, which is the same thing, of determining its 

 amount. This method he details at sufficient length to make it 

 intelligible to practical men. 



But though the correction of this deviation was the principal 

 object of Mr. Barlow's labours, he did not neglect to consider 

 the phenomena of magnetism under a scientific point of view ; 

 and he has made a discovery, which, if it prove correct, must be 

 admitted to be of first-rate importance, and will tend more to 

 bring magnetism into the state of an accurate science than any 

 fact respecting it yet brought into view. He has found that the 

 force of magnetism, like that of electricity, depends not upon 



