460 



Analyses of Boohs. 



[June, 



* Experiments to determine the Magnetic Deviation caused by a 

 galvanic vertical Wire at different Distances. Length of ver- 

 tical Wire 36 Inches. 



Deviation by 

 standard com- 

 pass. 



25° 0' 

 Ditto 

 Ditto 

 Ditto 



Distance of the 

 other compass 

 from the wire. 



12 inch. 

 8 

 6 



4 



Mean * obser- 

 ed deviation 



A. 



Value of- arc. 

 a 



— tan. — = A. 

 a 



5° 37' 



11 15 



16 30 



26 30 



18-772 

 34-100 

 47-712 

 77-500 



Mean 



Constant pro- 

 duct. A cot. A 



190880 

 171432 

 161062 

 154440 



164728 



" When it is considered that these observations were made on 

 a compass needle only one inch in length, and that the divisions 

 extended only to quarter points, it is impossible to expect a 

 closer approximation. The needle and card, however, being 

 delicately suspended, and the latter very distinctly divided, I 

 could depend upon my observations to the nearest degree ; for 

 by means of a strong magnifying power I could always bisect 

 and trisect the quarter points without any very sensible error." 



Mr. Barlow next gives some experiments to determine the mag- 

 netic deviation caused by a vertical galvanic wire ; the length 

 being varied, but the distance constantly nine inches ; and having 

 thus far verified his hypothesis by experiment, he proceeds to 

 the consideration of the deviation in different azimuths ; but as 

 the limits to which we are confined will not allow us to follow 

 him, we must terminate our notice of the section with some of 

 his concluding remarks. 



" My results," he says, " are necessarily only approximate ; 

 because I have throughout supposed the needle indefinitely 

 short in comparison with the distance and length of the wire ; 

 but by this means I have rendered the subject perfectly intelli- 

 gible to every one ; whereas had I taken the actual case of the 

 reciprocal action of every particle of the fluid in the wire upon 

 every particle in the needle, and had been able to complete the 

 investigation, it could only have been understood by a few 

 mathematicians; at the same time the minute corrections thus 

 introduced would not have been appreciable in the comparison 

 of the results with experiments ; these latter being necessarily 

 both liable to small irregularities, and difficult to observe." 



" It will have been noticed that I have only attempted to 

 illustrate the nature of the action which has place between agal- 



" " That is, the mean of two observations at each station of the compass ; the contact 

 being changed. The same is to be understood of the deviation with the standard com. 

 paw." 



