176 On the Magnetic Variation near Edinburgh. 



fording data, for estimating at some future period the progres- 

 sive change in this country, which is known to be going on in 

 the magnetic intensity. 



The instrument employed was a magnetic bar, capable of 

 being reversed on its axis, and which played over an arc placed 

 on a distant wall ; the true bearing of the zero having been ac- 

 curately determined by repeated transits of the sun. The in- 

 strument used was a simple magnetic bar of considerable length, 

 which was found to give decidedly more accurate results, and 

 to be free from sources of error to which smaller instruments 

 are liable. Care was taken to free the instrument from any 

 local cause of attraction, by having the pedestal at a distance 

 from accumulated masses of iron (which exist in all houses), 

 or volcanic rocks whose attraction likewise exerts considerable 

 influence on the needle ; and for this reason it may not be 

 advisable to compare the present observations with those made 

 at Edinburgh some years ago, where even at short distances 

 between the stations very different results were deduced.* 



There is also another subject to be considered in deducing 

 the mean variation, viz. the diurnal variation. From several se- 

 ries of observations continued at different periods with a needle, 

 delicately suspended by a silk fibre, it was found, that the hour 

 of the needle's mean position was 10 h 3 m A. M., that the needless 

 north pole then advanced progressively westward until 12 h 50 m 

 p. M. ; and then again receded to its mean place at 4 h 35 m P. M. 

 The total amount of arc passed over east and west of the mean 

 position from this cause was IS'. To compensate this error, 

 each series of observations was reduced to the mean hour. A 

 mean of these gave the following results, as the mean magnetic 

 variation for August 1836, at 



Inveresk, six miles east of Edinburgh, ) 2fi0 , 4fl// w 



Lat. 55 56' 20" N., Long. 3 2' 40" W. / = 26 



M ilnegraden Berwickshire, \ 2ro . / 9V/ w 



Lat. 55 41' 20" N., Long. 2 11' 41" W. / " 



Mr Bain, in his valuable Essay on the Variation of the Compass, gives 

 the following from his observations in 1814 at Edinburgh : 



variation. 



At the Observatory, 27* 50' 10" W. (120 obs.) 



... Grange's Park, West Church, . . 27 36 2 ... (30 ... ) 

 ... a Garden south side of Castle draw-bridge, 27 41 35 .. (40...) 



