26 REPORT — 1860. 



dip-circle by Robinson ; the following differential results were obtained without 

 reversing the poles of needle : — 



o / 



10th December, 1855, 6 h a.m. Base of hill 2 



( Top of hill. Circle \\ feet above rock A 3 34 



8- to io a.m. \ " ^ irc ! e i/° ot f " n !,; 



j „ Circle 4| feet „ B 1 30 



( „ Circle f foot „ 14 5 



Noon. Base of hill over laterite 1 50 



4 h p.m. Trevandrum Observatory 155 



The bearing of the Trevandrum Observatory was observed approximately by a 

 hand compass at the stations A and B. At 4^ feet above the rock the error was small ; 

 but when placed on the rock A, the decliuation was found 10° west, while on B 

 (9 feet W.N.W. of A) it was 35° east, the true declination over laterite being about 

 |° east. 



The pegmatite and granite on the top of the hill seemed to form kinds of dykes 

 running parallel to each other nearly north and south, and crossed by lines nearly at 

 right angles to the direction, so as to form large blocks, between which the decaying 

 rock has allowed the accumulation of soil to some depth. Blocks of about 9 inches 

 diameter were cut out of the rock at A and B, having previously marked the direc- 

 tion of the true north and south upon the upper surface, which was nearly horizontal. 

 With these specimens I made the following observations. A specimen was placed 

 with its centre at about 2 feet from the centre of the freely suspended magnet, and 

 in the line at right angles to the direction of the magnet ; the points of the compass 

 marked on the upper surface, when the specimen was in situ, were successively pre- 

 senled to the centre of the magnet, and the scale readings of the instrument were 

 observed. The direction of the plane of greatest force being found, the specimen was 

 inclined at different angles to the horizontal, till the direction of the line of greatest 

 force was determined. 



Specimen A : elliptic cylinder, axes 9f inches and 8 inches, average height about 

 5 inches; a granite containing a small quantity of hornblende, colour reddish grey; 

 from about 1 foot west of the position A for the dip observation. The numbers fol- 

 lowing are in scale divisions, each equal 15" nearly : — 



N. deflection +21-4 S. deflection -17-8 



N.N.E. „ -4-191 S.S.W. „ -17-3 



N.E. „ +153 S.W. „ -15-3 



E. , +0-8 W. „ - 2-3 



S.E. „ -12-3 N.W. „ +14-5 



S.S.E. „ -15-4 N.N.W. „ +19-9 



S. „ -17-8 N. „ +21-4 



The direction of the plane containing the magnetic axis is in this case nearly 

 north and south. On raising the point S. presented to the magnet, it was found 

 that the north end of the magnetic axis dipped from 10° to 20° below the south 

 horizon ; the exact position could not be determined, from the difficulty of keeping the 

 centre of the specimen always at the same distance from the magnet. The result 

 agrees with the fact that the dip observed on A was diminished, since the rock mag- 

 net having here its south end uppermost, would necessarily attract the north end of 

 the dipping needle. 



Specimen B : cylinder 10 inches diameter, 9 inches deep ; upper surface red and 

 weathered, interior bluish grey; contains besides the bluish felspar and quartz a 

 large quantity of hornblende. The observations for B were made only for the north 

 end of the axis. 



N.N.W. deflection -36-4 



N. „ -38-8 



N. by E. „ -440 



N.N.E. „ -42-4 



N.E. „ -36-0 



