444 M. A. Sccchi on the Periodical Variations 



observation. It may also be said that the small maxima in the 

 morning; and evening, which are in truth for the most part only 

 indicated, are only a portion of the regular period cut short 

 midway by the discontinuity introduced by the passage of the 

 sun from above to below the horizon, as we have elsewhere re- 

 marked in regard to the periods observed towards evening in 

 high latitudes *. Eor these reasons we have urged that a com- 

 plete explanation of the phenomenon depends on the law of the 

 distribution of magnetism on the globe. 



It may not be useless to state here what is habitually ob- 

 served at Bombay, that being a place situated to the north of 

 the equator, and in a latitude not very dissimilar in amount to 

 St. Helena, the lat. being 18° 53' 30'' N. From the observa- 

 tions made at this place and reduced by Mr. Montriou, an 

 oscillation results analogous to that of other countries; having 

 an eastern maximum a little before 8 h , and a minimum be- 

 tween noon and 1 p.m. Besides which, there are two other 

 small oscillations, one near sunrise, the other near sunset ; it is 

 evident that this is the nocturnal period interrupted by the in- 

 terposition of the earth. During the night the needle has a 

 very small oscillation. The horizontal force has a simple period, 

 but with disturbances which indicate the commencement of a 

 secondary period ; the vertical force shows a tendency to a like 

 ])eriod. When the observations have been continued for a 

 greater number of years, more certain results will be obtained. 

 See Obs. Magn. and Meteor, at the Obs. of Bombay for the 

 year 1847, part I. p. 493, and Plate 1. The Editor of the 

 Bombay Observations then concludes : — " The presence of the 

 sun seems to produce great magnetic variations in the day-time, 

 and it is otherwise manifest that it is not on account of the 

 heat only of that body ; for if this was the case, the curves of 

 the temperature would be similar to the magnetic curves. Be- 

 sides, the presence of the sun begins to be felt two hours before 

 sunrise, and lasts almost as long after sunset, so that the solar 

 magnetic influence appears to be quite independent of the tem- 

 perature of the place." 



But we are very far from denying that meteorological causes 

 may often affect the needle ; we know that every meteorological 

 change is accompanied by a change more or less marked in the 

 vapour of the atmosphere, and therefore by a development of 

 electricity. But when the needle is usually seen to complete 

 its regular oscillation tranquilly in the midst of the most violent 

 storms, and during tempests loaded with electricity, with 

 tremendous thunder and lightning, it may well be asked, What 

 are the conditions under ivhich electricity must develope itself in 

 * This appears to us to be the true explanation of the phenomenon. 



