]0-i MR BROUX ON THE RELATION OF THE VARIATIONS OF THE 



the morning maximum, and extends tlie other forward at all other hours of the 

 day, making the greatest strides at the time of the afternoon maximum. During 

 the second half of the year, it rests the previously advancing foot about an hour 

 before the evening maximum, and brings the lagging foot forward at the other 

 hours, but with the greatest rapidity at the time of the principal minimum. 



13. There is perhaps nothing more difficult in groping for tlie laws which regu- 

 late certain phenomena than the separation of the effects due to different causes ; 

 but it is quite obvious that, before we can arrive at any sound conclusion as to 

 simple laws, this must be done. In the determination of the diurnal period all 

 the observations at each hour for a calendar month or year are summed, and the 

 means taken ; in these summations are included several irregularities named dis- 

 turbances ; if the disturbances occurred equally i)ositive and negative at the same 

 hour, or were equally distributed over the twenty-four hours, a large enough series 

 of obsei'vations would serve to eliminate them ; neither of these suppositions seems 

 to hold, and accordingly, certain hours in some months are more affected by dis- 

 tm-bances than the same hour in other months, or than the other hours of the 

 same month ; the diurnal curve, therefore, is complex. There are other causes, 

 as will be seen afterwards, Avhich render it more so. 



li. In the attempt to determine whether the horizontal intensity varies with the 

 moon's declination, the days were numbered from the day of the moon's greatest N. 

 declination, counting that day till it returned to the greatest N. declination again ; 

 and, as 13 of the moon's revolutions, with regard to node, are equivalent to 12 

 lunations, and nearly to a year, the 13 revolutions, with regard to declination, were 

 selected for summation ; as, by this means, any effect due to varying phase, or to 

 annual period, woidd be eliminated. The mean intensity for each of the 13 days on 

 which the moon had its greatest N. declination were then summed together ; the 

 means for the 1 3 days numbered 1, in which the moon Avas moving south, and so 

 on. For the purpose of verifying the result thus obtained, similar summations 

 of the observations for 1845 were made ; in this case, however, only 12 revolutions 

 with respect to declination, were obtained, so that any effect of phase will not he 

 perfectly eliminated. No attempt has in either case been made to eluninate dis- 

 turbances. The results of these summations were projected, having previously 

 eliminated the effect of secular change. (See Cmwes, No. 6, Plate IV.) The 

 curve, from the observations of 184-1:, indicates a maximum about 2 days after 

 the moon has attained its greatest S. declination, and a maximum about a day 

 after it has attained its greatest N. declination — the maxima have nearly equal 

 values, so also have the minima. The branches ascending to and descending 

 from the period of greatest S. declination are greatest; so that the periods of mi- 

 nima are nearer, the greatest N. declination Ijeing a1)0ut 5 days before and after it. 

 The curve deduced from the observations of 1845 shews the maxima nearly at 



