LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 



13 



servations which should be subsequently made. The times of vibration commencing 

 with 40° are those inserted in this Table ; the series commencing with 20° will be 

 found in its due succession. 



This comparison having shown that the seven needles specified in the Table were 

 in a steady magnetic state, Captain Belcher despatched Nos. 7. 8. and 9. to England, 

 to have their times of vibration observed there, and to be returned to him on the coast 

 of California ; purposing by this means to attach his series of relative determinations 

 to the great body of results obtained by other observers. The needles were received 

 by me in August 1839, and were vibrated on the 12th and 13th of August at a suit- 

 able place near Woolwich, where I also observed the dip at the same time. They 

 reached Captain Belcher again in the following November at Mazatlan. In the 

 meantime the Sulphur had quitted Panama, having on board Nos. 5. 11. 12. and 13, 

 and had visited successively Cocos Island, Oahu one of the Sandwich Islands, Kodiack 

 and Sitka on the north-west coast of America, Fort Vancouver and Baker's Bay in 

 Columbia River, Port Bodega, San Francisco, Monterey, S*^ Barbara, San Pedro, San 

 Diego, San Quentin, San Bartholomew, Magdalena Bay, and St. Lucas Bay, arriving 

 at Mazatlan in November. At each of the above-named stations the times of vibration 

 of one or more of the needles were observed, and occasionally of all the four. On 

 the arrival of the three needles which had been sent to England, their times of vibra- 

 tion were observed, in comparison with the others, first at Mazatlan, and a few days 

 afterwards at San Bias, where, more time being available, the comparison was re- 

 peated on two different days, viz. on the 6th and 19th of December. From San Bias 



* Nos. 1. 3. and 4. are not included in this Table, because the observations on the 16th of March 1839 

 showed that they were still losing magnetism, and they were not therefore subsequently employed. No. 6. is 

 also omitted, and the cause is explained by a memorandum of Captain Belcher's to the following eflFect : 

 " No. 6. kept well during the first twenty-one months, and changed suddenly during an excursion to Conchagua 

 in November and December 1838. It was vibrated on the 20th and 22nd of November, and gave consistent 

 results : on the 27th it was carried on horseback up the Amapola hill, 3000 feet above the sea, and on its return 

 on the 22nd of December was found to have lost magnetism equivalent to upwards of 12 seconds in 510 seconds. 

 The surface rock on the Amapola hill was so highly magnetic that no satisfactory observations could be ob- 

 tained there with the needle." An examination of the subsequent observations of No. 6, compared with those 

 of the other needles, shows that its magnetism was unsteady for many months after this accident, becoming 

 gradually weaker. I have not, therefore, taken into account the observations with this needle, as they do not 

 yield independent results of equal value with the other needles, and there are enough consistent determinations 

 without them. 



