21 



least squares, determined the direction of the isodynamic line, in 

 the mean latitude of 3° 05' south, longitude 106° 4?'; where the 

 mean total force was 8*74'5. The angle which the isodynamic line 

 forms with the meridian is from north 85° 36' west to south 85° 36' 

 east, and increases southerly at the rate of '001 073 for each geogra- 

 phical mile. The line of least intensity, in the space over which 

 the Survey was carried, appears to agree almost exactly with the line 

 of no dip. 



To recapitulate briefly respecting the position of the principal 

 lines. The line of no dip cuts the meridian at an angle approach- 

 ing a right angle, and near the parallel of latitude of 8° north in the 

 longitude of Singapore ; the rate of progression being almost exactly 

 two miles of dip to one mile of latitude. The line of least total 

 force varies but little from the line of no dip, forming indeed a 

 somewhat larger angle with the meridian. The direction of the 

 maximum horizontal force forms an angle of nearly 65° with the 

 meridian from north-west to south-east, but neither the maximum 

 horizontal nor the minimum total intensity appear to be, strictly 

 speaking, lines. In taking two stations having the same value, the 

 one to the north and the other to the south of the position of 

 greatest horizontal intensity, and dividing the distance between 

 them, it might be supposed that this would be the position of the line 

 of greatest horizontal intensity ; but the maximum horizontal intensity 

 appears not to be a line, but a space or belt extending for a consi- 

 derable distance on either side of this line ; the value in this space 

 being 8*20. The minimum total force was 8'149; the station being 

 Madras, in latitude 13° 04' north, longitude 80° 16' cast, dip 

 7° 34' north, horizontal intensity 8*078, and declination 0" 56' 09" 

 east. 



In addition to the above, observations were made at sea, consist- 

 ing of temperatures of the air and sea ; the dry and wet bulb, the 

 standard thermometer, and, whenever practicable, of the dip with a 

 Fox's Dip Circle. The author remarks that it is impossible to over- 

 estimate the practical value of this instrument at sea in low lati- 

 tudes. The dip changes two minutes for every mile of latitude. 

 The seas are generally smooth. When astronomical observations 

 cannot be taken, the dip circle becomes a truly valuable latitude in- 

 strument, and in a few minutes, by day or by night, the latitude of 

 the ship can be correctly ascertained. 



The author concludes by remarking that he has found Colonel 

 Sabine's many works on magnetism of considerable use, more espe- 

 cially * Lloyd and Sabine's Magnetic Survey of Great Britain and 

 Ireland.' 



January 23, 1851. 



In consequence of the decease of the Marquis of Northampton, 

 who filled the office of President for eleven years, the Society did 

 not meet on this day. 



