MAGNETIC DECLINATION NEAR THE MAGNETIC EQUATOR. 671 



declination for each month. Mr Caldecott communicated to that Society the 

 fact shown in his curves, that the law of variation was, to a great extent, in- 

 verted in the course of October 1841. Although the fact that the law of diurnal 

 variation for December was nearly the inverse of that for June within the tropics, 

 must have been perceived early by the Directors of other intertropical observa- 

 tories, yet Mr Caldecott was, as far as I am aware, the first who brought the 

 fact with its proof to the notice of a scientific body .* 



As will appear hereafter, none of the intertropical observatories, excepting 

 that of Trevandrum, was sufficiently near the magnetic equator to approximate 

 to a solution of the question proposed by M. Arago ; and Cape Comorin, about 

 40 miles S.S.E. of Trevandrum, was one of the stations pointed out by this philo- 

 sopher for that end. 



Position of the Trevandrum Observatory. 



The observatory at Trevandrum is in latitude 8° 30' 32" N. (about 2°| S. mag- 

 netic inclination, and 26' E. magnetic declination in 1854), and 5 h 7 m 59 s , east of 

 Greenwich, on a height nearly 200 feet above the sea. The following results are 

 deduced from observations made under my direction, during the twelve years 

 1853 to 1864. 



Instruments used. 



Two declinometers were employed after 1853 ; one by Grubb of Dublin, 

 similar to that described in the introductions to the Makerstoun Observations — one 

 by Adie of London, made according to my own plans. With the former, all the 

 precautions were taken indicated for the Makerstoun declinometer ; and the tele- 

 scope could be compared with the south transit-mark of the observatory (4 miles 

 distant). The second was under a glass-receiver, from which the air was pumped ; 

 the suspension apparatus was supported by a tripod of glass rods, so that very 

 little metal was used in the construction of the instrument. A room was built 

 within the observatory for this declinometer, having a solid planked ceiling 

 covered like a terrace with plaster, several feet below the roof. The declinometer 

 was read from without the closed room, the telescope pillar forming part of a 



* Mr Caldecott gave the diurnal curve for the first half, and for the second half of October. 

 In Notes to the Observations, dated 10th November 1841, which were forwarded to the Royal 

 Society of London, in the same month, he deduced, — 



From the 1st half of October, max. of E. declin. 7 h a.m. ; min. h 28 m p.m. 

 „ 2d „ min. „ 8£ a.m. ; max. 4 h 28 m p.m. 



He attributed the change of law at the time to the monsoon, which broke out during the month ; 

 and he caused the observations to be continued every ten minutes, in the month of November, in 

 order to determine whether the change would continue. Mr Caldecott's remark was never published 

 by the Royal Society of London, but it will be found among the MS. of Trevandrum Observations, 

 in the archives of the Society, where I have myself seen it. It is necessary to add, that Mr Calde- 

 cott's observations, though affected by many errors (chiefly due to the imperfect construction of the 

 magnetometer boxes), were sufficiently exact for the determination of this change of law. 



