TRANSACTIONS OP THE SECTIONS. 9 



pass, the principal mass of the cylinder being below the level of the needle and card, 

 and its upper end being 2| inches above that level. 



By this means a deflection or deviation of 10° 10' was produced, the north end 

 of the needle being drawn that amount to the eastward of the correct magnetic 

 north. 



Cylinder No. 2 was next placed inside of No. 1, when the deviation was increased 

 to 12° 15'. 



Cylinder No. 3 was then placed inside of No. 2, and the deviation was again in- 

 creased to 14° 15', the north end of the needle being drawn to the eastward in each 

 case. 



Hansteen's magnetic force instrument was then placed with the centre of its 

 needle (as nearly as I could adjust it) in a similar position to that which the centre 

 of the compass had occupied, and the following reuults were obtained : — 



Time of 100 vibrations, starting 

 from an arc of 18°. 

 Previous to the cyhnders being brought into the observatory 6' 57" 



No. 1 cylinder in place 6 51 



No. 2 cylinder in place inside of No. 1 6 47 



No. 3 cylinder in place inside of No. 2" 6 45 



The force instrument being removed, a dipping-needle was then employed, and 

 the following are the results of the observations : — 



Mean of Readings. Dip. 

 Previous to the cylinders being brought into the observatory 68° 37 ' 



No. 1 cylinder placed to the south of the instrument 70 10 



No. 2 cylinder in place inside of No. 1 70 27 



No. 3 cyhnder in place inside of No. 2 70 37 



The conclusion to be deduced fjom all these observations appears to be, that to 

 the induced magnetism of the surfaces of each cylinder, acting upon each other, is 

 due a portion of the deviation ; and reasoning by analogy, a similar deduction is 

 applicable to the telescopic funnels of steam ships. 



Results of Experiments with three Iron Cylinders, showing their Effect upon the Com- 

 pass, the Dipping -Needle, and Hansteen's Magnetic Intensify Instrument, when 

 placed in given positions. Observers, Capt. Johnson and Mr. Brunton. 



Time of 100 vibrations of Hansteen's 



Intensity Needle, starting from an 



arc of 18°. N. 6' 57", 



Bearing of distant mark before the cylinders were 



brought into the observatory N. or 0° 0' 



No. 1, or largest cylinder in place to the eastward 



of the compass N. 10° 10' W. 6' 51" 



No. 2. Cylinder in place inside of No. 1 N. 12 15 W. 6 47 



No. 3, or smallest cylinder, in place inside of 



No. 2 N. 14 15 W. 6 45 



the north end of the needle being drawn to the eastward by the cylinders in each 

 case. The main body of the cylinders were below the level of the compass, and 

 their upper ends 2^ inches above the said level. 



Dip before cylinders were placed 68° 37' 



Dip No. 1, cylinder in place to the south of the needle 70 10 



Dip No. 2, cylinder in place 70 27 



Dip No. 3, cylinder in place 70 37 



Ft. in. 



Thickness of iron cylinder 0-^ 



Depth of cylinder 1 g 



Diameter of external or largest cylinder. No. 1 1 Q 



Card A of the standard "l Length of the two outer needles (each) 5 J^y 



compass J Ditto, two centre ditto 7^% 



Length of the needle in Hansteen's intensity instrument 2-fg 



Length of the dipping-needle 62V 



