17 



therefore the sun is to the south of these lines about 220 days of 

 the year. If we compare the oscillations of the declination at the dif- 

 ferent stations with the Singapore curves, we see that of the sixteen 

 stations two do not agree, one is doubtful, and the other thirteen do 

 agree with the march of the needle at Singapore. Of the three not 

 in accordance, the one that is doubtful is in the vicinity of the lines 

 of no dip and minimum force, whilst the two which do not agree, 

 viz. Madras and Moulmein, are both to the north of the line of no 

 dip, and of the line of minimum force. The Moulmein observa- 

 tions were made in the month of April ; the hours of morning maxi- 

 mum and minimum are 21 and noon, whereas at Singapore the hour 

 of morning minimum is 21, which is that of the maximum at 

 Moulmein. Again, the Madras observations were taken in the 

 autumn, the morning maximum and minimum being at 20 hours 

 and at noon ; there is no well-defined morning maximum at the 

 same period at Singapore, but the morning minimum occurs at 21 

 hours. We see, therefore, that the stations south of the line of no 

 dip and of minimum force, with the exception of Keemah in the 

 Celebes, all are in perfect accordance with the march of the needle 

 at Singapore. Sambooanga, in the island of Mindanao, is nearly 

 on the line of no dip, and agrees generally with the Singapore 

 curve ; but it is to be observed, that as we proceed northerly and 

 approach the lines of no dip and of minimum force, the similarity 

 to the Singapore curve becomes much more faint, although agreeing 

 tolerably well. This may be seen on inspection of the Plates, 

 where the curves of Sumatra, Java and the Cocos, which are ex- 

 treme southerly stations, agree exceedingly well with those at Sin- 

 gapore. 



The next observations referred to are those of the horizontal 

 force at different seasons. As the observations at the different 

 stations were made at different times of the year, it appeared ne- 

 cessary, as in the case of the declination, to ascertain what changes 

 might take place in the different seasons at a station where an un- 

 interrupted series had been observed for a considerable time. Ac- 

 cordingly, Singapore, where observations were made hourly during 

 three years, was chosen as the point to which all the stations of the 

 Survey were referred. Unlike the declination, which changes its time 

 of extreme western position, not only in every season, but in nearly 

 every month of the year, the maxiraun) horizontal force at Singa- 

 pore occurs with great regularity either at 22 or 23 hours, whilst 

 the minimum at 9 or 10 hours is but very faintly defined. In 

 other respects also the curves preserve a perfect parallelism, if we 

 except only the greater range during the spring. 



In the curves representing the changes of the horizontal force 

 at the different stations of the Survey a general similarity is per- 

 ceptible ; a very decided maximum at noon, with a minimum very 

 faintly marked. The irregularity, where observable, appears to be 

 principally attributable to the short time during which the observa- 

 tions were taken. At Moulmein, the large oscillation appears to be 

 principally due to the very great change of temperature. The coeffi- 



Proceedings of the Eoyal Society. Vol. VI. No. 77. 2 



