446 M. A. Secchi on the Periodical Variations 



All that we know with certainty on this subject is, like so 

 much besides, due to Colonel Sabine. He has collected the 

 principal results at which he has arrived from the discussion of 

 the Hobarton and Toronto Observations in a memoir inserted 

 in the Philosophical Transactions (March 1852), from which we 

 will give a short extract, as well to complete the exposition of 

 the laws of the magnetic changes, as to obtain some light for 

 guidance in future researches. 



A comparison of the Toronto and Hobarton observations esta- 

 blishes, that even the extraordinary perturbations, though oc- 

 curring at all hours of the day, yet when taken in a mass, have 

 a regular period, which depends on the local time, and have 

 opposite directions iu the opposite hemispheres ; so that the 

 perturbations which cause an easterly deviation at Toronto, cause 

 a westerly deviation at Hobarton, in conformity with the com- 

 plete magnetic antagonism at the two stations. This fact is 

 elicited without difficulty from the coincidence of the perturba- 

 tions observed at the two places in the same day, with a differ- 

 ence in local time corresponding to their difference in longitude. 

 The general result is, that easterly perturbations at Toronto and 

 westerly perturbations at Hobarton, have their minimum in 

 number and magnitude during the day and their maximum 

 during the night. This maximum occurs at Hobarton between 

 10 h and ll h , and at Toronto at 9 h . This difference in time, as 

 we have already remarked, occurs in all the other magnetic 

 changes. The minimum occurs at Hobarton between 5 and 6 

 a.m., and at Toronto between 2 and 3 p.m. The easterly per- 

 turbations at Hobarton and the westerly at Toronto, have a di- 

 stinct period. Their maximum at Toronto is at 5 a.m., and at 

 Hobarton at 6 a.m. ; the minimum at Toronto is between 9 

 and 10 p.m., and at Hobarton at 10 p.m. Taking the perturba- 

 tions in mass, and laying down the curve representing their 

 mean effect on the curve of the diurnal oscillation of the needle, 

 the following law is elicited : — " The morning perturbations tend 

 to diminish the ordinary excursion of the local period, and the 

 evening ones to augment it." 



This law may be enunciated in another way. " The pole 

 which is turned to the sun is by the mean effect of the pertur- 

 bations moved towards the east from 5 a.m. to 5 p.m. About 

 6 a.m. and 6 p.m. it passes zero, and the rest of the day is moved 

 to the west. The maximum movement in the morning is at 

 about 7, and in the evening at about 9. In both places a se- 

 condary minimum towards the west is observed at noon." In 

 other respects the curves are tolerably regular and of the usual 

 form ; but at Hobarton the principal maximum and minimum 

 are less marked than at Toronto; the march of the two curves 

 is in direct opposition at the two stations. These conclusions 



