MAGNETIC NEEDLE IN LONDON, IN AUGUST 1828. 51 



always with the longitudinal plane of the needle. The dip is then observed 

 and registered with the face of the instrument successively towards the east 

 and towards the west. The axis is then changed end for end, the nut that was 

 before screwed against the side 2 being now screwed against the side ; and 

 the dip again observed with the face of the instrument east and west. The axis 

 is then passed through the needle in a direction perpendicular to what it was 

 before, the nut being screwed, for example, against the side 1, and the dip ob- 

 served ; the axis is then changed end for end, and the dip again observed. 

 There are thus sixteen distinct observations in different positions of the axis 

 and instrument : this number is doubled by reversing the poles and repeating 

 the whole operation ; and as both ends of the needle are required to be read, 

 there are sixty-four arcs observed for each determination of the dip. In the pre- 

 sent case two distinct observations were made in every position of the needle, 

 axis, and instrument, and the dip is therefore an arithmetical mean of 128 ob- 

 served arcs. In the following abstract the reference to the sides implies that the 

 nut on the arm of the axis was screwed against that particular side of the cube 

 in the observations referred to it. 



August 13th & 15th. 3 to 5 P.M. Therm. 66°. 



f Marked end /Side 0; 70°52'.0 1 ,.„o . ./ q /Side 1; 70°33'.8 ") ^ o r./ o 



Exp.) aN.Pole 1 Side 2; 70 57 -7 J^ 1 Side 3; 71 16.8/'"^^"* 



IX. ) Marked end f Side 0; 69 44 .5 ") „„ .^ ^ / Side 1 ; 67 44 .0 1 „„ .^ „ 



I aS.Pole 1 Side 2; 67 52.6 /^^ ^^-^'1 Side 3; 69 52.0/^^ ^^-^ 



69 .51 .7< ^ )69 51 .65 



Mean .. 69° 51 '.67 



Captain Franklin having kindly obtained from the Colonial Department the 

 use of a small apparatus for detennining the dip which he carried with him 

 in his last land expedition, I am enabled to add the result of four series of ob- 

 servations made with it in the Garden at Chiswick, at the same time as the 

 preceding observations, by Mr. David Douglas of the Horticultural Society. 

 The circle is of six inches diameter only, being made so small for greater 

 portability : except in size, the whole apparatus is in all respects similar to 

 the one with which the observations already detailed were made. The agate 

 supports are levelled in the same manner by a circular plate carrying a 



H 2 



