101 



TERRESTRIAL MAGNETISM. 



TERRESTRIAL TEMPERATURE. 



its original position, and therefore determines the corresponding 

 variation of the magnetic meridian. 



Such is a brief outline of the methods employed in determining the 

 absolute value of the magnetic elements, and in watching the periodic 

 changes they are constantly found to undergo. It is obviously beyond 

 the limits of this article to enter into the numerous and elaborate 

 details of the cautions which must be adopted by the observer ; the 

 methods employed to diminish the effects of instrumental errors ; and 

 the corrections which are applied to many of the observations to 

 counteract the influence of temperature, which has the effect of 

 changing the magnetic condition of the bars. For a more complete 

 description of the magnetometers, and all the practical details necessary 

 for observing and computing the observations, the reader is referred to 

 the ' Instructions of the Royal Society to the Directors of Magnetical 

 and Meteorological Observatories.' 



We now proceed to notice some of the most prominent phenomena 

 in connection with terrestrial, magnetism. It has been already stated 

 that the magnetic elements are subject to periodic changes. These 

 changes are generally slow, and exhibit a certain amount of regularity, 

 though the laws which govern them have not yet been arrived at. One 

 of the most apparent, perhaps, of these fluctuations is seen in the 

 declination magnet, that end which is towards the north moving slowly 

 westward during the forenoon, and returning to its normal position 

 about ten in the evening. It then moves towards the east, and returns 

 to its former position about ten in the morning. These changes evi- 

 dently establish the fact that the motion of the magnetic meridian is 

 in some way connected with that of the sun. The other magnetic 

 elements also undergo similar though less striking changes ; and it is to 

 be observed that each succeeding day will not show exactly the same 

 set of hourly variations as its predecessor, nor are the observations of 

 one year identical with those of another. 



But, besides these known periodical fluctuations, which past expe- 

 rience teaches us to look for from time to time, there are sudden and 

 unaccountable disturbances in the magnetic elements arising, it is 

 supposed, from some sudden derangement in the magnetic condition of 

 the earth. 



This phenomenon exhibits itself in a sudden and sometimes violent 

 agitation of the suspended magnets. During these magnetic storms, 

 as they are called, the magnets oscillate to and fro, sometimes slowly 

 and regularly, at other times with such rapidity that the observer is 

 unable to note the time or arc of the vibration ; and often the scale by 

 which the position of the magnet is viewed is completely carried out 

 of the field of the telescope. On some occasions the magnet seems to 

 be acted upon by a succession of rapid jerks ; at other times a quivering 

 motion is detected, so much so as to render the scale indistinct. These 

 unusual disturbances, the particulars of which have, when observed, 

 been carefully recorded at the different observatories, do not appear to 

 have electricity as their origin, since they do not necessarily take place 

 during an electric storm. The most remarkable feature attending 

 them is the fact that simultaneous disturbances of the magnets at 

 different places are found to take place during a display of the aurora 

 borealis [TERRESTRIAL LIGHT], thus connecting that phenomenon 

 with terrestrial magnetism. 



Some philosophers have supposed that earthquakes are accompanied 

 by magnetic disturbances; but the great Humboldt doubts this, though 

 he was startled by observing, during the violent earthquake of Cumana 

 in 1799, " that the dip was diminished by 48'." 



Magnetic phenomena appear to be immediately connected with 

 meteorological changes. Hence it is that meteorology forms an 

 important part of the work of a magnetic observatory ; and observations 

 of the thermometer, barometer, wind, and weather are made simul- 

 taneously with those of the magnets. Violent gales, sudden changes 

 of temperature, and rapid barometric fluctuations are generally accom 

 panied by magnetic disturbances. 



Each place .on the earth's surface has its own distinct magnetic 

 elements. We have already stated that the freely suspended needle 

 does not point exactly north and south, but to two points near the 

 terrestrial poles. These points are called the magnetic poles, i When 

 the meridian of any place likewise passes through the magnetic poles 

 it is evident that the magnetic meridian coincides with the terrestrial 

 and there is no declination or variation ; and the locus of all such 

 places is called " the line of no variation." Again, at places near the 

 equator, the dip needle is horizontal, and the locus of all such places 

 which locus does not coincide with the equator, nor is it an exact circle 

 is called the " line of no dip." As we proceed towards either of the 

 terrestrial poles, the needle becomes more and more inclined to the 

 horizon, until we reach the magnetic poles, where it assumes a vertica 

 position. 



Likewise, also, the absolute intensity of the magnetic force depend 

 upon locality. It appears to increase as we proceed from the equatoi 

 to the poles, and the curves which pass through all those places where 

 the intensity is the same, are called " isodynamic lines." These line 

 do not coincide exactly with parallels of latitude, nor with lines of crjua. 

 '///. According to the most recent determination, the north magneti 

 pole is situated very nearly in latitude 70 6', and west longitude 98 47 

 Sonic have supposed the existence of two magnetic poles in each hemi 

 sphere, the position of the second northern pole being in Siberia, abou 

 irth latitude, and 102* east longitude. The positions which hav 

 ARTS ASD SCL PIV. VOL, VIII, 



een assigned to the southern poles are a little to the south of Australia, 

 nd south-south-east of New Zealand. This hypothesis, however, must 

 e adopted with caution. 



The following is a comparative view of the value of the magnetic 

 lements at some places where they have been determined with the 

 greatest accuracy : 



Declination. 

 Greenwich . . . 22 51' west 



Toronto . . . 

 St. Helena 

 Cape of Good Hope . 

 Hobart Town 



127' 

 22 46' 

 29 8' 



9 47' east 



Dip. 

 68 59' 

 75 )7' 

 21=37' 

 53 35' 

 70 36' 



Total Force. 

 10-4 

 13-9 



6-3 



7-6 

 13-5 



The causes of terrestrial magnetism have still to be developed. 

 Various theories have been from time to time proposed, some of which 

 lave indeed successfully accounted for a few of the kuown rnaguet- 

 cal phenomena, but fail under the tests which science and obser- 

 ation supply us with. The elaborate theory founded on the 

 upposition that the earth is an actual magnet, is overturned by the 

 liscoveries of Faraday, Arago, and other philosophers. Although 

 .here does not appear to be a simultaneous occurrence of magnetic and 

 electric storms, yet there can be little doubt of the connection between 

 :hese phenomena, even if magnetism be not "one of the numerous 

 brms under which electricity developes itself." The experiments of 

 Faraday and Oersted have shown, not only that " electricity induces 

 magnetism in the vicinity of the body which conducts it," but 

 ,hat " free magnetism gives rise to electricity." Assuming, what is 

 lighly probable, that the interior of the earth is a mass of liquid fire, 

 ;hen the idea of a magnetic nucleus in the earth must be abandoned, 

 'or it is found by experiment, that the magnetism of a body is de- 

 stroyed when it reaches a white heat. It is, therefore, the earth's 

 crust only which we may assume to be the seat of magnetic currents. 

 That such currents do exist, is by no means improbable, and experi- 

 ments seem to show that they are immediately excited by unequal 

 distribution of heat. If we regard the aurora borealis [TERRESTRIAL 

 LIGHT] as a luminous discharge of superabundant magnetism, we can 

 understand how this phenomenon should be confined to the vicinity of 

 the colder regions, where the earth's surface is least heated, and the 

 magnetic intensity, therefore, the greatest. No less than 146 years 

 ago, Halley (' Philosophical Transactions ') had boldly ventured to con- 

 jecture, that the terrestrial light was a magnetic phenomenon. But 

 when the fact became established, that the aurora indicated its 

 appearance by the disturbed state of the magnet in all parts of the 

 globe, and the brilliant discovery was made by Faraday, that light 

 could be evolved by magnetic power, then a passing conjecture ac- 

 quired a degree of probability amounting almost to certainty. 



It is curious that Galileo was inclined, to account for the parallel 

 direction of the earth's axis, on the supposition of a distant magnetic 

 point of attraction in space. Without entering upon the consideration 

 of a question which we usually refer to dynamical principles, yet it is 

 pretty certain from recent observations, that both the sun and moon 

 do exert an influence upon the magnet, and philosophers are strongly 

 disposed to connect the variable phenomenon of the spots on the sun 

 with the magnetic condition of our earth. 



For further and more complete information on this subject, and 

 subjects closely allied to it, the reader is referred to the following 

 authorities : Humboldt's ' Cosmos," Sabine's translation ; Somerville's 

 ' Connection of the Physical Sciences ; ' Young's ' Lectures on Natural 

 Philosophy ; ' Brewster's ' Magnetism ; ' Faraday's ' Researches ; ' 

 ' Reports of the British Association for the Advancement of Science, 

 on Magnetism and Meteorology ; ' ' Philosophical Transactions ' of the 

 Royal Society ; Barlow on ' Magnetic Attractions ; ' ' Annuaire Mag- 

 ndtique,' St. Petersburg, 1836 ; Gauss, on the ' General Theory of 

 Terrestrial Magnetism,' in Taylor's ' Scientific Memoirs,' August, 1839 ; 

 Gauss and Weber's 'Magnetic Atlas,' Leipzic, 1840; 'Report of the 

 Committee of Physics of the Royal Society ; ' ' Magnetical and Meteo- 

 rological Instructions of the Royal Society ; ' ' Magnetical and Meteo- 

 rological Observations made at different Observatories, and printed by 

 direction of the British and Foreign Governments, and the Honourable 

 East India Company.' 



Amongst those who have enriched this science by their investigations 

 and observations, we may make especial mention of General Sabine, to 

 whom, in con junction with Sir John Herschel, and the present Astrono- 

 mer Royal, is due the establishment of magnetic observatories and well 

 organised expeditions, which have furnished upwards of 2,000,000 of 

 observations in the course of three years, and who has, ever since the 

 year 1819, devoted himself to the cause of terrestrial magnetism. 



But terrestrial magnetism it still in its infancy ; much remains to be 

 doue ; and so simple and inexpensive are the methods and instruments 

 of observation, that many would find a delightful and profitable 

 amusement in the study especially in the observance of magnetic 

 storms and thus contribute their mite towards the discovery of " the 

 great ocean of truth." 



TERRESTRIAL TEMPERATURE, DISTRIBUTION OF. The 

 subject of the earth's temperature has already been treated of at some 

 length under different headings. Generally, it was shown under 

 CLIMATE and METEOROLOGY, that the temperature of the air in any 

 region depends on the inclination of the sun's rays to the surface of the 

 earth in that region ; on geographical position and physical conform*- 



H 



