1869.] Lieut. Elagiu's Determinations of the Dip. 283 



dips were found which differed from those obtained at other times about 

 seven minutes, whilst the other needle, A 2 , gave more uniform and satis- 

 factory results ; and this is also the reason I preferred to take the sepa- 

 rate means for each needle, and then means of both needles, and to give 

 to them equal weights, notwithstanding the number of observations is 

 greater in one case than the other. The cause of needle A l giving dif- 

 ferent positions must be most probably in the axis of the needle, not in 

 the agate plates ; otherwise both needles would indicate the same difference. 



Having given the results of my observations, I think it desirable to state 

 the precautions I took to obtain the best results. First of all, whilst at 

 the Royal Observatory, Greenwich, where I was for several months study- 

 ing the several instruments in the magnetic department, through the kind- 

 ness of the Astronomer Royal and Mr. Glaisher, I had made myself well 

 acquainted with the necessary care in those observations ; besides, I several 

 times visited the Kew Observatory, through the kindness of Dr. Balfour 

 Stewart, and took some observations of dip. At all times my first efforts 

 were directed to have a firm support ; next, to accurately levelling the 

 instrument; third, to see that the agate plates were clean, that the axis of 

 the needle was also clean and tested by the use of cork, that the needles 

 were free from dust and damp, their ends being passed in and out of cork, 

 and their surfaces wiped with wash-leather ; and in damp weather increased 

 attention w 7 as paid to everything ; but, as a rule, observations were not 

 made at such times ; care was also had in determining the magnetic 

 meridian corresponding, and in all cases several readings were taken in 

 every position. 



The results of observations of dip with local instruments at different 

 places were as follows : — 



Kew Observatory, monthly observations of dip with an instrument 

 No. 33 Circle, of the same pattern I had made by Barrow ; the length of the 

 needle is about 3| inches. To compare No. 33 Circle with the Circle bor- 

 rowed from Kew, I made simultaneous observations ; the mean from six 

 observations with two needles gave for No. 33 Circle = 6 8° 2' '19, and for 

 the Circle I had from Kew 68° 3' '8, this result being l'*6 larger. 



Royal Observatory, Greemvich. — Observations of dip are made frequently 

 with Mr. Airy's dip instrument, described in the yearly volumes of ob- 

 servations at the Royal Observatory. Six needles of three different 

 lengths are observed on the same instrument ; the results derived from 

 each separate needle seldom differ more than five minutes in the year. I 

 took from the Royal Observatory observations the mean of the determined 

 dip for the period from 1st July to 30th September, which was = 67° 56'* 1 5, 

 derived from twenty-seven observations, and nearly corresponds to the 

 time of my observations. The dip obtained from my observations with 

 Kew Circle was = 6/°58'-88, being 2'73 larger. 



Brussels Observatory. — The observations of dip were made with an 

 instrument of old English construction, which was made in the year 1828, 



VOL. XVII. Y 



