480 



General Sabine on Terrestrial Magnelism. 



[June 18, 



These results, when compared with the previous determinations for 

 these same instruments, would seem to show that a somewhat better result 

 is obtained when the exhaustion is carried on more slowly, and hence that 

 the corrections depend, to a considerable extent, on the nature of the treat- 

 ment received. No. 8 seems to be more constant under diiferent treatment 

 than No. 9. 



From ail these experiments we may perhaps conclude as follows 



(1) A good aneroid of large size may be corrected for temperature by 



an optician, so that the residual correction shall be very small. 



(2 a) If an aneroid correct, to commence with, be used for a balloon or 



mountain ascent, it will be tolerably correct for a decrease of about G inches 



of pressure. 



(2 /3) A large aneroid is more likely to be correct than a small one. 



(2 y) The range of correctness of an instrument used for mountain 

 ascents may be increased by a previous verification, a table of corrections 

 being thus obtained. 



(3 a) If an aneroid have remained some time at the top of a mountain, 

 and be supposed correct to start with, then it will give good results for 

 about 8 inches of increase of pressure. 



(3 /3) A large aneroid is more likely to be correct than a small one. 



(3 y) If the aneroid has been previously verified, it is likely to give a 

 better result. 



(4) After being subjected to sudden changes of pressure the zero of an 

 aneroid gradually changes, so that under such circumstances it ought only 

 to be used as a differential and not as an absolute instrument, that is to 

 say, used to determine the distance ascended, making it correct to begin 

 with, or to ascertain the distance descended, making it correct to begin 

 with, it being understood that the instrument ought to be quiescent for 

 some time before the change of pressure is made. 



Before concluding I ought to mention that most of the experiments 

 herein described were undertaken and executed in a very careful manner by 

 Mr. T. W. Baker. 



XVI. "Contributions to Terrestrial Magnetism, No. XI.'' By 

 General Sabine, R.A., P.E.S., &c. Received June 18, 1868. 



(Abstract.) ' 



This number of the Contributions of Terrestrial Magnetism contains the 

 completion of the Magnetical Survey of the South Polar Regions, under- 

 taken by Her Majesty's Government in 1840-1845 at the joint instance 

 of tbe Royal Society and the British Association for the Advancement of 

 Science. The observations themselves, and their provisicnal discussion, 

 have already been given in the previous nurni eif, Y., YI., Till., and X. 

 of the Contributions. The present number contains a general review of 



