136 Notices respecting New Books. 



traversing platform, by means of wliicli it could be traversed, or 

 carried round, in a complete circle. These experiments con- 

 firmed all his preceding deductions, showing that the plane of 

 no attraction, and all the laws and circumstances attending re- 

 gular masses of iron, obtained also on those the most irregular; 

 and consequently that they might be immediately applied to de- 

 termining the local attraction of the guns, &c. on shipboard. 

 The comparison is accordingly made between the observed devi- 

 ations on board the Isabella, as given by Captain Sabine, off 

 Shetland, and those computed on the above principles ; and the 

 coincidence between them is very striking, considering all the 

 circumstances of the case; viz. the change in the dip, tiie very 

 irregular distrilnition of the iron in the ship, the particular situa- 

 tion of the compass, in the vessel in question; and lastly, that 

 the laws themselves had been inferred fiom experiments made in 

 one latitude only, and on masses of iron of the most regular figure. 



One very serious objection may however be made to this, as 

 well as to any other r2ile for computing the quantity of deviation 

 iiv different latitudes ; which is, that the dip at the place of ob- 

 servation is a necessary datum, and it is one that is not readily 

 obtained. This induced Mr. Barlow to look for some other me- 

 thod for determining the error in question, the theory of which 

 may be explained in few vvords. 



Since it was rendered obvious from his experiments on the 

 2^ -pounder, that all the action of the mass might be referred to 

 one common centre of attraction; it followed, that so also in a 

 ship, the whole might be reduced to one centre of four; and since 

 the iron and compass will always, during the voyage, preserve the 

 same or nearly the same relative situation, it is assumed, that a 

 single ball of iron might be so pointed in the ship, that its ac- 

 tion on the needle would !>e the same as that of the iron in its 

 distributed state ; or, whicli is still the same, that there is one 

 common residtant. Thi ? being admitted, it follows from the laws 

 previously established, that if a line be conceived to be drawn 

 joining that centre with the pivot of the needle, it will be possi- 

 ble to introduce a less ball of iron, having its centre of attraction 

 in the same line, (which is to be approximated so much nearer as it 

 is less,) that shall produce exactly the same efl^ect as the great ball 

 at a greater distance, or as the iron in its distributed state. So 

 that, whatever effect the guns, &c. may produce in any situation 

 of the vessel, the above ball being placed in its fixed situation, 

 that effect will be doubled ; whence the quantity of deviation it- 

 self becomes determined. 



To be a little more explicit: A vessel having got her guns, 

 Sec. on board, before she sails, is to be warjjcd round, and the 



devialioi; 



