SABINE'S MAGNETIC OBSERVATIONS AT TORONTO. THIS volume enables us to
trace the history of the great British magnetio survey now in progress, with more of accuracy of detail than was possible when we last had occasion to advert to the subject.* A magnetic sur- vey of the British Islands was commenced in 1834, and carried throne), in the two following years, by the joint labours of five members of the British Association for the Advancement of Science. The results were published in 1838, in the annual volume of the Association's reports. In the same year, a memorial addressed to Government by the Associa- tion was favourably received, and gave occasion to the Naval Expedition of 1839-43 for a magnetic survey of the high latitudes of the Southern hemisphere. In the spring of 1837, the University of Dublin had voted funds for the establishment of a magnetic and meteorological observatory; in the summer of 1837, a site Was allotted by Government for a magnetic observatory at Greenwich, to be placed under the superintendence of the Astronomer Royal; and a third magnetic observatory has since been established in Britain, (near Kelso, in Scotland,) by the private munifi- cence of Sir Thomas MacDougall Brisbane. • Fixed magnetic observa- tories were in 1840-41 established under the departments of the Ord- nance and Admiralty, at certain stations of prominent magnetic interest within the limits of the British Colonial dominions,—in Canada and Van Diemen's Land, as approximate to the points of greatest intensity of the magnetic force in the Northern and Southern hemispheres ; at St. Helena, as approximate to the point of least intensity on the globe ; and at the Cape of Good Hope, as a station where the secular changes of the mag- netic elements presented features of peculiar interest. In 1839, the Directors of the East India Company, at the request of the Royal Society, ordered the establishment of magnetic observatories at Simla, Singapore, Madras, and Aden.
This brief recapitulation presents to us a bird's-eye view of a system
of magnetic observation commenced with a degree of zeal and on a scale of extent worthy of the high character of British science and the re- sources of the British state. What was so well begun has been followed up on the whole with corresponding perseverance and energy. The va- luable observations at the British Magnetic Observatories are periodically given to the public. The observations made at sea by the Antarctic Expedition have been reduced and published by the Royal Society in the Philosophical Transactions. The reduction and publication of those made at the Colonial Observatories was intrusted to Colonel Sabine, assisted by Lieutenant Riddell and four military clerks. The observa- tions made on days of unusual disturbance in the years 1840-41 were published in 1843, for the purpose of comparison with each other, and with similar observations made simultaneously in different parts of the globe. The volume now before us contains the magnetic and meteorological observations made at the Toronto Observatory during the years 1840-41-42; and those made at the other Colonial Observatories are preparing for publication with all possible speed. The observations made by the officers of the East India Company's Engineers are trawl- mitted to the Royal Society as they arrive in England : steps will doubtless be taken forthwith by that distinguished body for their reduc- tion and publication.
It is obvious that a correct magnetic theory can only be obtained by
such a laborious and sustained system of observation as we have been describing: and the importance of a correct magnetic theory to the greatest maritime nation that exists or ever has existed is equally obvious. The following remarks by Colonel Sabine on the diurnal variation of the declination of the needle at Toronto, at once warrant a hope that our ob- servers arc on the traces of a general law, and show the necessity of perseverance in observation. The mean diurnal variation of the declination at Toronto, as derived from the two-hourly observations of the two years 1841-42, consists in an Easterly move- ment of the North end of the magnet from 2 to 10 hours inclusive ; a small re- turn movement towards the West then taking place till 14 hours; when the Easterly progression is resumed, and continues until 20 hours, at which time the North end of the magnet reaches its Eastern limit. From 20 hours the movement is continuous towards the West until 2 hours; which is the period of the extreme Western limit. If we examine the variation separately shown in the Spring,• Summer, Autumn, and Winter quarters of each of the two years, we find that tne alternate progression and retrogression, twice in the twenty-four hours, is mani- fested sevecilly in all, though with somewhat less regularity in the turning - points during the nocturnal hours than when the sun is above the horizon. This inferior degree of regularity during the night occasions in two of the eight quar- ters under notice the occurrence of a triple maximum and minimum; and in a third quarter, the substitution of 12 for 10 hours as the time of the nocturnal Eastern limit. In the remaining five quarters, the turning.hours in the night are the same as those of the means of the two years. The hours of the day at which the Easterly and Westerly limits are respectively: attained are the same in each of the quarters as in the general mean; except in the Winter quarter of 1841, when the forenoon Easterly limit occurs at 22 instead of 20 hours. The range of the diurnal variation is throughout greater in the Summer than in the Winter months; this is particularly the case with the Easterly movement from 14 to 20 hours, and the subsequent return. The progressive increase in the extent of this movement may be traced from the mid-winter, when it is barely perceptible, to the mid-summer, when it is very considerable. In consequence of this inequality, it occasionally happens in the mid-winter months, that the evening Eastern limit is the Eastern extreme of the whole diurnal range, which is formed at other periods by the morning Eastern limit. When we compare the corresponding quarters of 1841-42 with each other, and the general mean in 1841 with that in 1842, we perceive on the one hand a very satisfactory degree of general accord- ance; but, on the other hand, we must remark such partial or occasional differ- ences as show that a longer period than two years is required to give numerical values at the several observation-hours, which we can regard AS possessed of a sufficient degree of precision to be adopted as final conclusions." It appears, then, that the pulsations of the magnet when observed at one place for two whole years are regularly periodical ; their slight diversities seeming to indicate increased sluggishness of motion at the hours and sea- Spectator, No. 802; llth November 1843. sons of the greatest -privation of light and heat. The observations on un- usual dishirhanoes show, that when the regular pulsations of the magnet are affected by what have been called "magnetic storms," the disturbance is simultaneous at Van Diemen's Land and Canada. Such facts encourage to perseverance in search of a great pervading law, which once discovered Must impart greater confidence and security to all who take the magnet f6r their guide. Numerous incidental discoveries, all practically useful, will be made in the course of the search after this law. Take, for ex- ample, the following application of the established maximums and minimums of Easterly delIntione.
"We may derive- from this itiveetigation the following practical inferences for the instruction of persons who may have occasion to employ the compass in sur- veying and other similar pupates in Canada—lst., That large deviations from the mean monthly direction of the needle at the same hours arc least likely to occur from noon to 4 in the afternoon: 2nd, That at all other hours of the day the liability to the occurrence of deflexion exceeding 5 or 6 minutes from the same direction at the same hours, is about three times as great as at the hours of 2;and 4 p.m. The liability, salt may be estimated from the two-hourly observe- time, is about one ohservatiou in twenty-eight, and at 8 and 10 p.m. rather greeter: 3rd. That the disturbances are usually deflexions, ef the North end of the needle to the West in the forenoon, and to the East front 6 p.m. to midnight inclusive."
There is another point of view in which this magnetic survey appears most important. We find the Government and various scientific bodies, the Artillery of the Queen's. Army, the Engineers of the Company's ser- vice, the Navy, and the University of Toronto, cotiperating to promote the survey. It is a great national object, imparting a community of interest to various classes of men in the Mother-country and in the Colonies. In iteelf a pursuit adapted to elevate and refine the mind, it is at the same time one of those thousand links which form the chain of national feel- ing. No fewer than twenty non-commissioned officere of the Artillery have been employed upon those observations, and have displayed extraordinary aptitude for scientific acquiremente. The practice of occu- pying the men with such pursuits is an important step in the education of their class.
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