departme:nt of terrestrial magnetism. 157 



Besides the magnetic work proper, the question of sufficiently accurate 

 geographic positions for the points of observations has received further 

 special attention by the consulting hydrographer, Mr. G. W. Littlehales, and 

 the commander, Mr. Peters. Various astronomical methods of observation 

 and of computation were subjected to careful investigation. 



The oceanic magnetic zvork accomplished thus far may be briefly summar- 

 ised as follozvs: 



The aggregate length of the present Cruise III, up to arrival at Honolulu 

 August 28 last, is about 22,400 nautical miles, and as the two previous 

 cruises (1905 and 1906) had a combined length of about 26,000 miles, the 

 Galilee, during the period of August i, 1905, to September i, 1907, has 

 cruised in all nearly 50,000 miles over regions in the Pacific Ocean where 

 before there were but very few carefully executed magnetic observations. 



A complete determination of three magnetic elements (magnetic declina- 

 tion, magnetic inclination, and intensity of magnetic force) has been secured, 

 on the average about every 200 or 250 miles along the entire route. Plate 10, 

 showing the various cruises, will give a better idea than words can convey 

 as to what has actually been accomplished, as also how far the state of our 

 knowledge of the distribution of the magnetic forces over the Pacific Ocean 

 will have been advanced when the Galilee has cruised over the regions from 

 Honolulu to New Zealand and from thence through the South Pacific and 

 has returned to San Francisco next year. 



In addition to magnetic results on the ocean, the Galilee has obtained a 

 number of highly important results on the islands and ports where stoppages 

 were made, some of the points being reoccupations of former stations, and 

 has besides made a series of valuable intercomparisons of instruments at the 

 various observatories visited. 



All the results obtained by the Department at sea and on land up to March 

 of the present year were furnished the U. S. Hydrographic Office in time to 

 enable it to issue last May a new "Chart of the lines of equal magnetic varia- 

 tion for 1910" (lines of equal magnetic declination), representing not a little 

 improvement in the lines over the Pacific Ocean. Along some of the well- 

 traversed routes in this ocean, errors in the existing charts amounting from 

 1° to 5° were disclosed. These are corrections of considerable importance 

 to navigators, to say nothing of the advancement of our knowledge along 

 purely scientific lines. 



The values of the magnetic dip as given over the North Pacific Ocean by 

 the existing charts have been found to be, in general, too small by about 

 1° to 3°, and the values of horizontal intensity too high by about one twenty- 

 fifth part. 



With the aid of the data furnished, the U. S. Hydrographic Office has in 

 preparation also "Chart of the lines of equal magnetic dip" and a "Chart of 

 the lines of equal magnetic intensity." It would hardly be advantageous 



