298 Analyses of Books. [Oct. 



these laws hold in irregular masses of iron. For this purpose 

 our author applied to Sir William Congreve for permission to 

 pursue his inquiries in the Royal Military Repository at Woolwich; 

 which was immediately granted. He selected for his purpose an 

 iron 24-pounder, mounted on a platform, which admitted of its 

 being traversed through an entire circumference ; the trucks at 

 the bottom running over a circle ten feet six inches in diameter. 

 With this gun he made a set of experiments, which agreed with 

 the precednig ones in every essential particular, and showed, 

 that the same laws hold in the case of irregular masses of iron, 

 as were ascertained to exist in iron balls. 



In the 12th section, Mr. Barlow explains the metluod which 

 he conceives calculated to enable us to determine the deviation 

 occasioned by the iron on ship-board in all situations of the 

 vessel, and in every part of the world. The method is simply 

 this : Procure an iron plate of such dimensions that when placed 

 •within a certain distance of the compass, it will just double the 

 angle of deviation. Ascertain this position, and mark it; then 

 set the plate of iron aside. When at any time it is wished to 

 ascertain the angle of deviation, we have only to observe the 

 position of the compass, and then place the iron plate in the 

 place marked for it. The angle of deviation will now be doubled. 

 Hence the deviation produced by the plate will mark the devia- 

 tion of the compass from the true point. In reality indeed it is 

 only the tangents of these two angles that are equal ; but as in 

 small angles the tangents are very nearly proportional to the 

 angles, we may assume the angles as equal, without any mate- 

 rial error. Mr. Barlow, however, has given a formula for calcu- 

 lating the true angle of deviation from the tangents. 



The 13th section is taken up with a set of directions for apply- 

 ing this corrective apparatus to the binnacle of a vessel. These 

 directions must be of great importance to practical seamen, 

 though they w^ould not be sufficiently interesting to the general 

 reader to make it requisite for us to give them a place here. We 

 understand that Mr. Barlow is at present employed verifying 

 this method, by actually trying the experiment on board several 

 men of war. We trust that the method will be found satisfactory 

 in practice. It will furnish a most valuable addition to the 

 improvements in navigation which have been made during the 

 courseofthe lastcentury; and oua'ht greatly to facilitate the disco- 

 very of the law by which the declination of the compass is regu- 

 lated. At least it will enable navigators to determine the true 

 declination with much greater facility than formerly. 



The 14th section is occupied with a hypothetical explanation 

 of the daily variation of the compass. It is now well known that 

 the declination of the needle not only varies from year to year, 

 but even, during every day, a small variation may be perceived 

 by means of a delicate instrument. Let us suppose the needle 

 at sunrise to have a particular position ; as the sun advances 



