1837.] Dip and Intensity at Madras. 375 



N. B. The numbers 1,2, 3, &c. exhibit the order in which the ob- 

 servations were made. During the present century, I cannot find 

 that any observations for Dip have been made at Madras, but there 

 is one result on record dated 1775, when AbercroSibie found it to be 

 5° 15' N. ; if this result can be trusted, it would appear that the Dip 

 is on the increase at the rate of 1' 34" in a year. 



"With regard to the needles employed for the magnetic intensity, it 

 may be necessary to state, that they are constructed after the model of 

 that of Professor Hansteen. The needles are cylinders, 2^ inches long 

 and .3 inch in diameter, save that the ends are abruptly sharpened to a 

 point ; these needles are freely suspended on their centres by a few fila- 

 ments of unspun silk, which are hooked on to a brass stirrup, moveable 

 upon the needle ; by which means a perfect adjustment to horizontality 

 can be effected ; the needle thus suspended is enclosed in a rectangular 

 glass box immediately over a divided circle, from which the arc of 

 vibration can be read off and the number of oscillations counted. 

 The zero of measure here employed, is the time of performing 100 

 vibrations at a temperature of 60", commencing with an arc of 20° 

 and ending at from 2* to 4°. — If these measures could be observed to 

 ultimate accuracy, it would be worth while to reduce the times of 

 vibration under these circumstances to the times of describing an in- 

 finitely small arc, as has been done by Hansteen, and on account of 

 buoyancy, to a vacuum ; but since such is not the case, the result will 

 be obtained to all useful accuracy by supposing the correction common 

 to each set of observations, by which the reductions, which are rather 

 operose, are avoided : the reduction to a temperature of 60° is effected 

 by applying the correction, 0,00017 t. (where t represents the time 

 of performing 100 vibrations) ; — a formula which is derived from ex- 

 periment. The two needles used in the following observations are dis- 

 tinguished from one another by a sign X on one of them. This needle 

 in London at a temperature of 60° performed 100 vibrations in 442,76 

 seconds of mean time, whereas the other needle performed 100 vi- 

 brations under the same circumstances in 461,96 seconds; the former 

 needle is further distinguished from the latter from its having been 

 long in use in England, and as having exhibited a remarkable degree 

 of steadiness in its magnetism during the late magnetical experiments 

 instituted in Ireland under the auspices of the British Association; 

 added to which, these needles are calculated to excite a more than 

 ordinary degree of interest from the circumstance of their having 

 been employed by Sir John Ross in the perilous North Polar Expedi- 

 tion, from which he has lately so fortunately returned. The observa- 

 tions at Madras are as follows. 

 3 c 2 



