196 Observations of the Dip of the Needle 



[No. SO. 



, An inspection of Table No. 2 will shew the Dip observed 

 at the two different periods, the change of Dip in the interval 

 and also the Latitude of the Magnetic Equator deduced from 

 the Formula 



tan fft== 2. tan. \ 



S being the Dip and \ the Magnetic Latitude of the place. 



The mean change of Dip during the interval of about 

 years thus appears to be about 34' 48" consequently there must 

 have been an average annual increase of about 6/6". 



L^pon comparing this result however with that obtained 

 from a series of stationary observations taken at Madras .dur- 

 ing the last 2\ years it would appear to be too large. This 

 series is given in Table 1 and calculating the change of Dip 

 from the observations in corresponding months of successive 

 years we get a mean annual increase of only 2'. 



In order that we may form some judgment as to which of 

 these two results is most worthy of credit it will be necessary 

 to state the circumstances under which the observations were 

 severally taken. 



The observations made at Madras form an uninterrupted 

 series they were all taken with the same Needle and circle 

 and in the same place. Each monthly result moreover is the 

 mean of 16 observations, 2 being taken on the Tuesdays and 

 Fridays of each week. 



The large result on the other hand is deduced from a com- 

 parison of 2 different series of observations taken with differ- 

 ent Instruments and Needles and under totally different cir- 

 cumstances. Moreover the Needles used in 1838 were cer- 

 tainly inferior to those at present employed and the Magnets 

 used in Magnetising them being of a small size, could not so 

 thoroughly saturate them as the powerful ones now employed. 



