TRANSACTIONS OF SECTION E. 709 



Magnetic Charts. 



Of the valuable work of tlie several fixed magnetic observatories of the world ,_ 

 I may remark that they are constantly recording the never-ceasing movements of 

 the needle, the key to many mysteries to science existing in the world and 

 external to it, but of which we have not yet learnt the use. Unfortunately many 

 of these once fixed observatories have become travellers to positions where the 

 earth can carry on its work on the needle undisturbed by electric trams and 

 railways which have sprung up near them, and it is to be hoped they will find 

 rest there for many years to come. 



Of the forty-two observatories which publish the values of the magnetic 

 elements obtained there, thirty-two are situated northward of the parallel of 

 30° N., and only four in south latitude ; and it is a grief to magneticians tbat so 

 important a position as Capetown or its neighbourhood does not make an additional 

 fixed magnetic observatory of the first order. 



Thus, as far as our present question of magnetic charts and their compilation ia 

 concerned, the observatories do not contribute largely, but we should be very 

 grateful to them for the accurate observations of the secular change they provide 

 which are so difficult to obtain elsewhere. 



Of the value of magnetic charts for difi'erent epochs I have much to say, as 

 they are required for purely scientific inquiry as well as for practical uses. It is 

 only by their means that we can really compare the enormous changes which 

 take place in the magnetism of the globe as a whole ; they are useful to the 

 miner, but considerably more so to the seaman. Had it not been for the charts 

 compiled from the results of the untiring labours of travellers by land and 

 observers at sea in the field of terrestrial magnetism during the last century, not 

 only would science have been miserably poorer, but it is not too much to say that 

 the modern iron or steel steamship traversing the ocean on the darkest night at 

 great speed would have been almost an impossibility, whereas with their aid 

 the modern navigators can drive their ships at a speed of 26"5 statute miles an 

 hour with comparative confidence, even when neither sun, moon, nor stars are 

 appearing. 



Of the large number of travellers by sea, including those who embark with the 

 purpose of increasing our geographical knowledge of distant lands and busying 

 themselves with most useful inquiries into the geology, botany, zoology, and 

 meteorology of the regions they visit, few realise that when they set foot on 

 board ship (for all ships are now constructed of iron or steel) they are living 

 inside a magnet. Truly a magnet, having become one by the inductive action of 

 that great parent magnet — the Earth. 



How fares the compass on board those magnets, the ships, that instrument so 

 indispensable to navigation, which Victor Hugo has forcibly called ' the soul of 

 the ship,' and of which it has been written, 



' A rusted nail, placed near the faithful compass, 

 Will sway it from the truth, and wreck an argosy ' ? 



And if so small a thing as an iron nail be a danger, what are we to say to the 

 iron ship ':' Let us for a moment consider this important matter. 



If the nature of the whole of the iron or steel used in construction of ships 

 were such as to become permanently magnetic, their navigation would be much 

 simplified, as our knowledge of terrestrial magnetism would enable us to provide 

 correctors for any disturbing efi'ects of such iron on the compass, which would 

 then point correctly. But ships, taken as a whole, are generally more or less 

 unstable magnets, and constantly subject to change, not only on change of 

 geographical position, but also of dii'ection of the ship's head with regard to the 

 magnetic meridian. Thus a ship steering on an easterly course may be tem- 

 porarily magnetised to a certain extent, but on reversing the ship's course to west 

 she would after a time become temporarily magnetised to the same amount, but 

 in the opposite direction, the north point of the compass being attracted in each 

 case to that side of the ship which is southernmost. 



