On the Secular Change of Magnetic Declination in Canada, from 1790 to 1850

By Edward Taylor Fletcher

[Originally published by the Literary and Historical Society of Quebec in Transactions, New Series, No. 3 (1865)]

(Read before the Society, 17th May, 1865.)

It has occurred to me that an examination of the various Returns of Survey of record in the Department of Crown Lands might be, to some extent, useful in furnishing additional data for the solution of that interesting problem in magnetic science which proposes to determine the limits, causes, and elements of the secular deflections of the magnetised needle. It is obvious that reliable observations in different parts of Canada, if obtainable, would be of great value, from the immense extent of the colony from north-east to southwest, nearly at right angles to the presumed direction of the magnetic isogonal lines. The records of the Department I have mentioned, comprising, as it does, the old Surveyor General's Offices for Upper and Lower Canada, and constituting, with its other formerly independent branches, rather a federation of offices than one Department alone, reach back for a full century; nay, even further, if we include the ancient seigniorial concessions and various special land descriptions of a date anterior to the conquest. As a surveyor attached to the Department for the last four and twenty years, I could not be blind to the possibility of some good from this source, though a closer examination of the older records has, it is true, somewhat lowered the expectations I originally entertained by shewing me how little, comparatively, of substantial and reliable fact was obtainable from these voluminous files, and that for these reasons :—In the first place, by far the greater part of the older surveys were performed by the needle alone, without reference to the true or astronomical bearing; in these, therefore, there was no mention of magnetic variation or declension; the only exception was where the side-lines of seigniories were concerned, which, by the old French arrêts or rėglements, were required to have a fixed astronomical bearing—that is, due northwest and south-east on the St. Lawrence, and nord-quart de nord-est, or north 12¼° east, on the Ottawa. In these cases the surveyors were compelled to lay off a true meridian, and thus the variation was not unfrequently noted. In the second place, some surveyors seem to have carried the variation with them from one part of the country to another, and, having compared their circumferentor with the meridian stones set up at Quebec and elsewhere, to have travelled off with the variation thus obtained as a fixed and unchangeable element, good for all parts of the district. Thus the surveyor intrusted, some seventy years ago, with the survey of the Yamaska, the Richelieu, or the Chateauguay rivers, might possibly start from Quebec by batteau (for as yet steamers were not) for Three Rivers, and, having spent perhaps a week or ten days in the voyage, if the wind was unfavorable, would, on landing at the latter town, compare with every possible minuteness and care the magnetic bearing shewn by his circumferentor with the true north and south line given by the meridian stones. This done, he formally notes in his field-book or journal: "je trouve donc que la variation de non instrument est de dix degrès trente minutes;" and this ten degrees thirty minutes goes with him throughout an operation of perhaps many miles in extent as a constant and unalterable companion. Yet far be it from me to decry the merits of these patriarchs of the profession. Though sometimes a little shaky in theory, in practice they were frequently admirable: I have myself had frequent occasion to verify the accuracy of their work, and the journal of their operations shows a wonderful dexterity, quick-wittedness, and fertility of resource under difficulties. At times on the verge of starvation, or annoyed by the wild animals which at that time swarmed everywhere, or running dangerous rapids on small rafts, and losing perhaps some one of the party, they seem to have been always prompt, fearless, and uncomplaining, knowing their duty, and striving, like honest I men, to perform it faithfully and well. Other causes of unsatisfactory observations are to be found in the entire ignoring the diurnal and annual variation, and in the imperfectness of the instruments themselves. Local attraction and disturbances have also been a frequent cause of error. As a proof of the liability to error from observed declination, where the diurnal or annual

variation is not taken into account, I would only instance the annual variation, greatest, as is known, in June and July, and least in December and January. From the records of the Toronto observatory it appears that in January, 1841, the observed declination was 1° 11' 1"; in June and July of the same year it was 1° 17'; while in the following year, 1842, the declination was in January 1° 14', and in June and July 1° 20'. Thus the declination: at Toronto, though increasing from year to year, is yet greater in June and July, 1841, than in January, 1842.

The observations of later days are free from, many of these errors. The instruments now in use arc of improved make and more accurate construction. The causes which influence the local and periodic disturbances of the needle are better understood and more clearly appreciated. The course of study, also, now required from candidates for the profession, is of wider scope "than formerly. A reference to the 77th chapter of the Consolidated Statutes will shew the various branches of physical and exact science which form part of the course. Bach candidate is subjected to two examinations: one on mathematical subjects alone, before being indentured; and a second, embracing all subjects, at the expiration of his term, and before receiving his diploma. There appears, therefore, reason to believe that in future, as now, magnetic observations of this kind may be received as, in general, reliable.

Towards the close of the last century, fixed meridian stones were laid down at Quebec, Three Rivers and Montreal. They are frequently referred to in the old returns of survey, and must have been of great use in determining the secular charge of declination in those several localities.

On eliminating with care from the observations of record, giving priority of place to those which were obtained by direct observation of Polaris or of the Sun, or by comparison with the meridian stones, the following facts appear to be established:—

There has been a fluctuation or oscillation in Canada, as elsewhere, of the amount of magnetic declination. At Quebec, this horizontal deflection, which in 1649 is said to have been 16° W., and in 1686, 15° 30', was in 1785, 12° 35', and in 1793, 12° 5' according to Major Holland, Surveyor General at the time. In 1805 it appears to have reached its minimum, being then only 11° 35' in the same locality. In 1810 it had increased to 12° 30', I and since then it has either been stationary or slowly increasing up to the present day, when it is over 16 degrees. From 1821 to 1831 it appears to have remained unchanged at 13°. In 1840 it had reached 13½°; in 1845 it was about 14½°; in 1850 it may be set down as 15½°, and at the present day it is stated to be 16° 40'.

In Toronto the rate of increase appears to be about three minutes annually since 1840.

These fluctuations are susceptible of explanation on the well-known hypothesis of Hansteen, the Norwegian professor, who found that the supposition of two northern and two southern magnetic poles would best accord with the observed phenomena of terrestrial magnetism. Of these two northern poles one is stronger than the other, the ratio of intensity being about as 17 to 10. There is also a stronger and a weaker southern pole. The place of the stronger north pole was in 1790, in W. longitude 95° 37'and in N. latitude 70°. The weaker north pole was then in E. longitude 127° 30' and in N. latitude 85° 30'. In 1820 the stronger north pole was six degrees farther east, or in W. longitude 89° 24', while the weaker north pole had meanwhile moved nearly thirteen degrees further east, and was in E. longitude 140° 6' and in N. latitude 85° 12', and finally in 1850, the stronger north pole was in W. longitude 83° 10' and latitude 69° 14', while the weaker pole had moved forward to 150° 40' of E. longitude and 85° of N. latitude. thus both poles move towards the east, but the weaker much more rapidly. Having therefore given the position of these poles sat any time, and their relative intensity, it is not difficult to compute, by the doctrine of the composition of forces, the precise; place of the resultant pole or that resulting from the two forces combined, and to which may be referred, as the true and absolute magnetic pole, all the observed phenomena of deflection. It will then be seen that this resultant pole sometimes moves to the east, sometimes to the west, and at others appears to pause and remain stationary for several years in succession.

In connection with this subject, I submit also a table of the magnetic declination at Quebec for several years, from 1649 for two centuries onwards, and also a table of Lower Canada declinations towards the close of last century. These tables are compiled almost wholly from the field-books, plans and procès -verbaux of surveyors, and from other official records. Among the most valuable materials are the recorded amounts of declination which in old times each surveyor found his circumferentor or theodolite to shew when compared with the meridian stones already adverted to.

I have also consulted a list of Upper Canada declinations, obligingly communicated to me by Mr. Russell, the Assistant Commissioner of Crown Lands, to whom the profession is in many ways largely indebted; but of these, the older observations appear to be so much at variance, and so hard to reconcile, that I have thought it best to abstain for the present from any publication of its contents, until I could see a little more clearly into the matter.