March 27, 1884] 



NATURE 



499 



as compared with those observed in middle latitudes, both 

 in respect of the turning hours and in the direction of the 

 movement at the same local time. Furthermore, investi- 

 gation of observations made by Polar voyagers and Arctic 

 travellers had shown that the northern part of these 

 British possessions was a region of peculiar interest as 

 comprising in its area the most powerful of the two foci 

 of magnetic intensity in the northern hemisphere, and 

 also the locus of vertical dip commonly recognised as the 

 North Magnetic Pole. 



To Lieut. Lefroy — furnished with transportable mag- 

 netometers — was assigned the arduous and responsible 

 duty of traversing this region of such striking magnetical 

 interest, to determine the absolute values of the declina- 

 tion, inclination, and intensity at available stations ; and 

 at one or more fixed winter residences in high latitudes 

 to make hourly and term day observations of those 

 regular and irregular fluctuations in the movements of 

 the needle presumed to exist in values of more than 

 ordinary magnitude. 



Sir Henry Lefroy's present volume contains the diary 

 of his journeys — these latter extending to 5480 geogra- 

 phical miles— in which is given in more or less detail the 

 magnetical elements determined at three hundred and 

 fourteen stations, combined with such astronomical ob- 

 servations as were necessary, in the then imperfect state 

 of the maps of the region traversed, to approximately assign 

 the geographical positions of the places of observation. 



The extended series of hourly and term day magnetical 

 and meteorological observations made at the fixed winter 

 and spring residences [1843-44] of Fort Chipewyan on 

 Lake Athabasca (lat. 58° 43' N., long. 111° 19' W.), and 

 at Fort Simpson on Mackenzie River (lat. 61° 51' N. 

 long. 121° 25' W.), with their very complete and able dis- 

 cussion by Capt. Lefroy, were printed by order of Her 

 Majesty's Government in 1855. This masterly work is 

 well known to those interested in the science of terrestrial 

 magnetism. The Diary now for the first time published 

 is a fitting sequel to the earlier work ; and is not the less 

 valuable from what may appear to be its tardy produc- 

 tion. The author's preface — which conveys a graceful 

 tribute to his old chief — in a few words clears up the 

 seeming anomaly. He says: — 



" The renewed attention directed to the distribution 

 and periodical changes of the earth's magnetism in the 

 North Polar region, suggests an endeavour on my part to 

 present the observations of my magnetical survey of 

 1843-44 with fuller explanation, and in a form more con- 

 venient for reference than that in which they were origin- 

 ally published '' [Sabine's ' Contributions to Terrestrial 

 Magnetism,' No. vii. Philosopliical Transaciions, 1846, 

 and No. xiii. Philosophical Transactions, 1872]; "and 

 being still the principal authority for the received position 

 of the focus or pole of greatest magnetic intensity, as well 

 as for the lines of equal magnetic force, equal inclination, 

 and equal variation over a large part of the continent of 

 North America, it is certain that whenever they come to 

 be repeated, the observer of the future will inquire for 

 particulars not contained in, and not suitable for, the 

 Philosophical Transactions" 



Apart from the value of a full record of the observa- 

 tions made over so great an extent of continental America, 

 is the consideration of the graphical treatment of the iso- 

 clinal and isodynamic lines, as embodied on maps accom- 

 panying the Diary. The author dwells on the difference 



in rendering the mapped results on the system followed 

 by Sabine — as given in the Philosophical Transactions — 

 and on that adopted by himself. Sabine's aim was to 

 present, over the vast area he was dealing with, the normal 

 values of the magnetical elements, free — so far as his 

 judgment permitted — from the local disturbances experi- 

 enced at individual stations, depending on geological 

 conditions ; and stations at which the disturbances 

 obviously affected the assumed normal values were 

 designedly rejected. Lefroy's treatment includes every 

 station at which he set up his instruments, rejecting no 

 observation because of its anomaly where there was no 

 internal appearance of error. By the one investigator we 

 are thus presented with a harmonious mapped system of 

 r^ular lines or curves indicating equal values of the 

 magnetic elements ; by the other the equivalent lines of 

 equal values are more or less sinuous, in some places 

 much distorted, and losing the semblance of regularity. 



In the case of the isoclinal lines as so developed, the 

 author partly infers that their greater inflections bear 

 some relation to the courses of the rivers ; and he further 

 draws attention to his isodynamic lines or curves, differ- 

 ing both in form and position from those of Sabine. 



In a discussion as to the appositeness of either of the 

 systems pursued, it must be borne in mind that, at 

 numerous well-known points on the earth's surface, a 

 movement made by the observer of the magnetic needle 

 a foot or two vertically, or a few feet horizontally, either 

 way, considerably aftects the observations. This is 

 notably the case at many oceanic islands, and a marked 

 example is to be found on our own coasts at Canna near 

 the island of Skye. Sir H. Lefroy's experiences in this 

 direction are well marked at Stations LIl. and CXXL, 

 wheie the total force observed was i5';6and I5'38 respec- 

 tively; the normal value undoubtedly was about i4'io 

 and I4'i5 ; the disturbance from a local geological cause 

 thus increased the total force by i/i4th. It is therefore 

 certain that, unless we have some fairly approximate 

 knowledge of the normal value of the magnetic elements 

 at the disturbed station, we should remain in ignorance 

 of the extent of the disturbance. 



In the present state of our knowledge of the distribu- 

 tion of magnetism in the several determinate values of 

 declination, inclination, and intensity over the earth's 

 surface — limited in the best explored regions to a very 

 small number of points of observation compared with the 

 great areas of land and water which they represent — it 

 appears premature to give interpretation to local disturb- 

 ances as being connected with topographical features 

 rather than geological. On general grounds we must 

 consider the delineation of the normal lines in any region 

 as a primary need, whether in a theoretical or a practical 

 direction. 



Local magnetic disturbances demand a special study ; 

 this has been given effect to in a theoretical direction by 

 Lament in Germany (" Researches on the Direction and 

 Intensity of Terrestrial Magnetism in Northern Germany, 

 Belgium, Holland, and Denmark in the Year 1858," 

 Munich, 1859), and practically is being now worked out 

 in the United States ; it is understood a special magnetic 

 survey of the State of Missouri is nearly complete. 



As magnetical observations multiply over large areas 

 of land, it is possible that the normal lines may be found 



