336 Colonel E. Sabine on the Magnetic Variation in 



the view of introducing the use of these instruments more exten- 

 sively in our mercantile marine, — an object which may be further 

 promoted by making Captain Klein's opinion known to the 

 readers of the Philosophical Magazine. 



But the particular object of this communication is not so much 

 to give additional currency to the Captain's commendation of the 

 Kew barometer, as to notice a remark of his which occurs sub- 

 sequently in the same printed letter, regarding maps of the mag- 

 netic variation. The remark is as follows : — " It was surprising 

 to see how incorrect the variation charts are : my results differed 

 often greatly from Raper's approximations, and I should fre- 

 quently have wavered had I not invariably obtained the same 

 amount as was indicated by the last observations made on land." 

 With respect to one of the authorities to which Captain Klein 

 refers, viz. " Raper's approximations," I have of course nothing 

 to say ; I am not myself acquainted with the work ; but as being 

 myself the author of a recent Map of the Variation over the 

 North and South Atlantic Oceans, I could not but be solicitous, 

 on reading Captain Klein's remark, to obtain a knowledge of 

 any inaccui'acies within the limits of that map which might 

 appear to be substantiated by his observations ; and having un- 

 derstood that the observations had been placed in the hands of 

 Dr. Buys Ballot, Director of the Marine Meteorological Depart- 

 ment in Holland, I wrote to that gentleman requesting him to 

 communicate to me any particulars contained in them which 

 might appear to be at variance with the variation lines in my 

 map, to which I knew Dr. Buys Ballot had the means of referring. 



I may here notice, that as the accuracy of my map rests wholly 

 on the accuracy of the observations which were coordinated in 

 its construction, I can have no motive whatsoever for not being 

 glad to receive corrections, if more accurate observations are pro- 

 duced than those which were at my command when the map was 

 constructed ; and that for the express purpose of obtaining such 

 corrections (should any appear to be required) by means of the 

 officers of the Dutch Navy, who have the general reputation of 

 being careful and skilful navigators, I presented several copies 

 of my map a short time after it was published to Chevalier Swart, 

 Hydi'ogi'apher of the Dutch Navy, for distribution among Dutch 

 officers. The chief peculiarities in this map which might be con- 

 sidered to add to its practical value, were, that it was constructed 

 in all its parts for one and the same epoch, viz. 1840; that no 

 other observations than those of contemporaneous, or very nearly 

 contemporaneous date, were employed in its construction ; that 

 all the observations on which it was based were engraved on the 

 face of the map ; and that a memoir was attached to it in which 

 these observations were discussed, and theii' corrections, either 



