134 Notices respecting New Books* 



pass. Such, however, appears to have been the case ; and Mr. 

 Barlow has in consequence the honour of having discovered se- 

 veral important laws which promise to throw considerablie light 

 upon this mysterious subject. 



After a few minor trials, this gentleman procured from the 

 Royal arsenal a solid 13-inch ball weighing nearly 300 lbs. ; 

 and having constructed a proper apparatus, whereby he could 

 place the compass at any required distance from the centre of the 

 ball, and in any proposed situation, he commenced Iris course 

 of experiments, and was soon led to the discovery of a very re- 

 markable fact, namely, that if we conceive a plane to pass through 

 the centre of any iron ball, inclining from the magnetic north to 

 south, and forming with the horizon an angle of about 20°, a 

 compass placed any where in that plane will not be atfected by 

 the iron, but will point duly north and south, tha same as if no 

 iron were in its vicinity. It occurred to Mr. Barlow afterwards, 

 that as the dip in this country is at the present time about 70''4, 

 the inclination of the above plane of no attraction, if examined 

 a little more particularly, mi^ht be found to be exactly the com- 

 plement of the dip; and subsequent experiments, on an improved 

 apparatus, and on an excellent dipping needle, confirmed this 

 conjectuie ; from which coincidence the author is led to infer, 

 that the same obtains in all latitudes ; namely, that the plane of 

 no attraction is every where perpendicular to the natinal inclina- 

 tion of the dipping needle. We have spoken of the plane of no 

 attraction as passing through the centre of the ball in a direction 

 perpendicular to the inclined needle ; but it is obviously the same, 

 if we suppose the plane to pass through the pivot of the compass, 

 s^nd the centre of attraction of the iron to be situated in that 

 plane; which latter conception is to be preferred in all cases where 

 irregular masses of iron arc employed. 



This very remarkalde fact being once well established, Mr. 

 Barlow made it the ground-work of all his subsequent experi- 

 ments: for example, he very naturally conceived a circle to be 

 described in this plane concentric with the pivot of the needle; 

 and conceiving at the same time an imaginary sphere to be cir- 

 cumscriijcd about the latter, this circle became to him an 

 equator; tiie line joining the extremities of the dipping needle, 

 the poles ; and the circle passing through those poles, and the 

 east and west paints of the horizon, a first or principal meridian; 

 by means of wiiich every particular situation on the sphere might 

 be immediately indicated by its latitude and longitude, the same 

 as on the tcriestrial surface. 



A long course of experiments were now undertaken, in order 

 to discover the law of deviation, as it depended upon the position 



of 



