612 



influence of that portion of the observed diurnal variation which 

 proceeds from the disturbances ; and he now adduces the observa- 

 tions at Point Barrow as strongly confirmatory of this. When the 

 diurnal variation is derived from the whole body of the observations 

 at Point Barrow, retaining the disturbances, the westerly extreme of 

 the diurnal excursion, which, as is well known, occurs generally in 

 the extra-tropical part of the northern hemisphere at a little after 

 1 P.M., is found to take place at 11 P.M.; but when the larger 

 disturbances are omitted, the westerly extreme falls at the same 

 hour as elsewhere, viz. a little after 1 P.M. The author takes 

 occasion to suggest the probability that the anomalies which have 

 been supposed to exist elsewhere in the turning-hours of the solar 

 diurnal variation in high latitudes may be susceptible of a similar 

 explanation. 



It appears, therefore, by the comparison of the Point Barrow and 

 Toronto observations, that in the regular solar diurnal variation the 

 progression at the two stations is similar, the easterly and westerly 

 extremes being reached nearly at the same hours ; whilst in the 

 disturbance diurnal variation the progression is reversed, the easterly 

 extreme at the one station coinciding very nearly with the westerly 

 extreme at the other. This contrariety seems the more remarkable, 

 since both variations appear to have the same primary or exciting 

 cause, viz. the sun ; inasmuch as in each the period is a solar day. 

 The author draws the probable inference, that whilst the primary 

 cause is the same in both, the mode of operation is different in the 



Another important distinction between the phenomena of the 

 solar diurnal variation and of the disturbance variation at Point Bar- 

 row and Toronto, is shown by the author to consist in the comparative 

 magnitude of their range. The increase in the range of the solar diurnal 

 variation between Toronto and Point Barrow is, as nearly as may be, 

 in the inverse ratio of the values of the horizontal force of the earth 

 at the two stations (which is the antagonistic force opposing all 

 magnetic variations) ; whilst on the other hand the increase in the 

 range of the disturbance variation is many times greater than it 

 would be according to the same proportion. It would appear there- 

 fore that the absolute disturbing force must be much greater at Point 

 Barrow than at Toronto ; suggesting the question, by what physical 



