364 



known influential cause, to permit the laws of the disturbing action 

 to be investigated ; and when these laws are known, we are furnished 

 with the means of making at least an approximate estimation of the 

 influence exercised by the uneliminated minor disturbances on the 

 laws which we may proceed to deduce for the class of observa- 

 tions from which we have not been able to effect their perfect 

 separation. 



Adopting this method of partially eliminating the influence of the 

 magnetic storms in the observations at Hobarton and Toronto, and 

 proceeding in the first instance with the caution suitable to a first 

 experiment, an unnecessarily high value (as it subsequently proved) 

 was taken as that which should distinguish a perturbed observation, 

 and consequently but a small body of disturbed observations was 

 separated. On a recalculation of the diurnal variation after the 

 elimination of these, and a comparison of the results with the diurnal 

 variation obtained previously from the whole of the observations, the 

 character of the influence of the magnetic storms was very manifest. 

 By the elimination of the larger disturbances, the interruption to a 

 continuous progression from the afternoon of the one day to the 

 morning of the following, was considerably diminished both in con- 

 tinuance and amount. A smaller separating value was then taken, 

 and consequently a larger body of disturbed observations was 

 eliminated ; the effect produced was a still further reduction of the 

 nocturnal feature. These first essays were sufficient to show that 

 the mean effects of the magnetic storms on the declination magnet, 

 both at Hobarton and at Toronto, attained a maximum *in the early 

 hours of the night, and constituted at both stations a very con- 

 siderable part, if not the whole, of the nocturnal portion of the 

 double progression which has been described. By still further 

 diminishing the separating value, but still keeping it well within the 

 limits in which no complication of disturbing causes would be 

 hazarded, so little was found to remain of the nocturnal interruption, 

 that I ventured, in the 1st volume of the < Toronto Observations,' 

 published in 1845, to express the opinion that "if the whole in- 

 fluence of the magnetic storms could be eliminated from the obser- 

 vations, the residual portion of the diurnal variation would be a 

 single progression with but one maximum and one minimum in the 

 twenty-four hours." 



