MAGNETICAL INSTRUCTIONS. c> 



of these duties over an extended area is not without value; the following Memoranda supplementary 

 to the article in the "Admiralty Manual of Scientific Enquiry" have been collected by the officer in 

 charge of the magnetic department of this office, Captain Evans, R.N., F.R.S., in great part Erom the 

 results of the expeditions to the Antarctic regions in 1839-43 in H.M. Ships " Erebus " and " Terror" 

 under Sir James Ross, and in the " Pagoda " in 1845 ; to be found in the several " Contributions to 

 Terrestrial Magnetism " by Sir Edward Sabine, published in the Philosophical Transactions of the 

 Royal Society, between the years 1841-70. 



G. H. bun \i;ns. 

 Htdrographic Office, Admiralty, 

 December 1872. 



Magnetic surveys, whether on land or at sea, are considered to include, for the particular station 

 or spot, the three component magnetic elements, namely, the Intensity or Force, the Inclination or 

 Dip of the needle, and the Declination or Variation of the compass. 



With regard to the general instructions for the determination of the magnetic elements on land, 

 nothing can be added to those laid down in the " Admiralty Manual of Scientific Enquiry," except to 

 enforce the preliminary caution (par. 10) of avoiding the neighbourhood, when possible, of rocks of 

 igneous character* sandy beaches where iron particles abound, and buildings or localities where 

 concealed iron above or under ground may possibly exist : — and especially to observe due care that 

 when two magnetic elements are being observed at the same time the respective instruments may be 

 so far separated as to prevent disturbance, and that the packing cases of the instruments containing 

 spare magnets, are also removed to a suitable distance. That keys, penknives, steel buckles, &c, be 

 removed from the dress of the observer, and that when the element of time is required magnets are 

 not placed near the chronometer. 



Magnetic observations made at sea differ from those made on land in the essential particular that 

 they are subject to the disturbing influence of the iron of the ship, and these disturbing causes, even 

 when reduced to their smallest practicable amount and kept as far as possible from the magnetic 

 instruments, still exercise a disturbing influence which in high latitudes becomes excessive, and under 

 any circumstances connected with the ship's geographical position, requires to be met by appropriate 

 corrections. 



On the Measurements of the Magnetic Intensity or Force. 



"With reference to the measurements of the magnetic force it must be remembered that they are 

 of two kinds (1) those designed to determine the absolute value of the horizontal component of the 

 force ; and (2) those which have for their object to determine the ratio of the total force at different 

 stations. 



For the absolute determination of the horizontal component, the unifilar magnetometer is 



* St Helena, Ascension, Trinidad in the South Atlantic ; the Auckland group, south of New Zealand, present 

 familiar examples of disturbing rocks ; many of the sandy beaches in New Zealand are largely formed of iron sand. 

 The presence of disturbing rocks or other influeaces, can alone be determined by preliminary observations over the 

 doubtful locality. 



VOL. II. B 



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