299 



It is formed on the principle suggested by Gauss, of separat- 

 ing the needle altogether from the circle by vvhich its position 

 is determined ; the moveable arms of the divided circle being- 

 furnished with compound microscopes, by means of which the 

 extremities of the needle are observed. The circle is divided 

 to 10'; and the readings are made to single minutes by the 

 help of verniers. 



By a simple modification in the construction of the in- 

 strument, it is capable of being used with needles of various 

 lengths, not exceeding the diameter of the measuring circle ; 

 and it is hoped that it will thus serve to determine the ques- 

 tion, not yet solved, as to the most advantageous dimensions 

 of the dip needle. 



Another, and more important, peculiarity in this instru- 

 ment, is its adaptation to the determination of the intensity in 

 absolute measure. In the received method, the horizontal 

 component of the intensity is determined by a double observa- 

 tion, and the total intensity thence inferred by multiplying it 

 by the secant of the inclination. Hence, in the high mag- 

 netic latitudes, where the inclination is considerable, any error 

 in it will induce a large error in the inferred intensity ; and 

 in the neighbourhood of the magnetic pole the method fails 

 altogether. The present instrument is adapted to measure the 

 total intensity directly ; and it is easily shewn that the result 

 thus obtained is, in the high magnetic latitudes, much more 

 accurate than that deduced in the ordinary way. The instru- 

 ment may be employed, also, for the measurement of the hori- 

 zontal and of the vertical components of the intensity. 



Mr. Clibborn read a notice of certain Bronze Antiquities 

 in the Museum of the Academy. 



Mr. Clibborn stated that he had lately detected iron 

 cores in the centre of the bronze composing the mouth- 

 pieces of several ancient horse-bits in the Museum. The 

 fact was interesting, as it appeared to prove that, at the time 



