36 



Mr. J. A. Broun on Siiimltaneous 



[May 11, 



For all tlie cases, as well as for these, the minima occur more nearly 

 simultaneously at all the stations than the maxima. 



When the various local causes (includiug the clilferent annual and 

 diurnal variations) which may accelerate or retard the epochs at different 

 stations are taken into account, we may conclude that the maxima and 

 minima occur nearly simultaneously over the area embraced in this in- 

 vestigation ; and it ma_y be added that they are as nearly simultaneous 

 as the daily mean maxima and minima o£ the horizontal force of the 

 earth's magnetism for different stations on the earth's surface. (See 

 plate 28, vol. xxii. Trans. Roy. Soc. Edin.) 



It has been already stated that other causes of change of atmospheric 

 pressure enter in higher latitudes than those here considered ; how far 

 the changes agree with each other, and how far they differ from those in 

 low latitudes, ^ill be the subject of another note. It was found that at 

 St. Helena the movements now studied were much smaller in general 

 than in India, and sometimes in opposite directions*. I have, however, 

 chosen the well-marked movement, March 31 to April 5 (which has 

 nearly the same amplitude at St. Helena as at the Indian stations), 

 for comparison. The 24-hourly means for the four stations are pro- 

 jected, Nos. 4, 6, 7, and 8, Plate 1. The epochs for this movement 

 have been already given (17 and 18). We find from the curve No. 4, 

 Plate 1, 



At St. Helena, the maximum Mar. 31*^ b^, the minimum Apr. 3^ I''. 



Thus the maximum occurred about 3 hours earHer than at Singapore, 

 and the minimum about 6 hours later than at Singapore and Madras by 

 the local hours t. 



The amplitudes of the different movements have not been taken into 

 consideration ; it is obvious from the curves, p. 28, that they are on the 

 whole greatest at Simla and least at Singapore. The barometric falls 

 at the four stations, March 19 to March 24, are :— 



in. 



Simla 0'27 



Madras 0-27 



Singapore 0-17 



St. Helena 0'06 



* Comptes Eendus, t. 75, p. 10. 



t It should be observed that the hours employed are in all cases the local hours for 

 each station ; and though, on account of the epochs being sometimes earlier and some- 

 times later at each place, the mean intervals are not affected for the three Indian 

 stations, where the greatest difference of longitude is not 2 hom-s, yet at St. Helena 

 there is a difference with Singapore of nearlj- 7 hours ; so that if the time is counted 

 from a common first meridian the intervals would be, for St. Helena, the maximum 4 

 hours, the minimum 13 hours later than at Singapore. 



