JiLY 10. mo:!.] 



SCIENCE. 



lid 



of the dilHeulty of making: an accurate 

 hydrographic survey in regions where the 

 coral rocks rise in pinnacles from relatively 

 great depths with appalling suddenness. 

 As a concrete example take a small area 

 about -400 S(iuare miles lying between Porto 

 Rico and St. Thomas, a region used by our 

 fleet for its maneuvers, and consider what 

 it means to find with the lead every hidden 

 coral rock or reef which might cause the 

 destruction of a seven-million-dollar battle- 

 ship. 



Not only must the depths be correctly 

 .shown, but as a further aid to navigation 

 the characteristics of the bottom must be 

 indicated, and there are places on the At- 

 lantic coast where the nature of the bot- 

 tom, as disclosed by the material which is 

 brought up by the sounding-lead, is so 

 characteristic of the particular locality that 

 it tells the navigator the exact position of 

 his ship. 



"\Mien the triangulation and topography 

 are complete, and the channels and general 

 configuration of the bottom have been de- 

 veloped and charted in their true relation 

 to the natural or artificial objects on shore 

 which guide the navigator, yet is the chart 

 not complete. The rise and fall of the tide 

 as affecting the indications of the chart 

 must be known at any time in present and 

 future. To the difficult problem of the 

 tides the survey has also addressed itself, 

 and permanent stations are maintained 

 which record automatically the stages of 

 the tide. The information furnished by 

 them is supplemented by shorter series of 

 observations made at intermediate places 

 by our own surveying parties or by others. 

 The connnerce of this country, however, 

 knows no geographical l)oundaries, and the 

 survey collects and publishes annually in 

 advance a vohnne giving predictions for 

 nearly all the ports of the world. 



Another branch of the survey which 



covers a broad field of observation and re- 

 search is that of terrestrial magnetism, 

 represented on the chart by compass dia- 

 grams. In order to draw them correctly 

 and by means of them to show the amount 

 of the variation of the needle at given lo- 

 calities, the magnetic elements have been 

 investigated from the earliest days of the 

 survey. At first these investigations were 

 inaugurated in the interests of the mariner 

 alone, and confined to the neighborhood 

 of the coasts, but as years passed the de- 

 mands made on the survey for more in- 

 formation required their extension to the 

 whole ai-ea of the United States and be- 

 yond. The intimate relation of the com- 

 pass to property surveys is the chief case 

 in point. The rerunning of the boundaries 

 of old estates, the interpretation of old 

 deeds in litigation, require a knowledge ot 

 the amount of the needle's variation in the 

 present time and the means of computing 

 its direction in the past. In the more deli- 

 cate work of the electrical engineer the 

 earth's magnetic elements have also to be 

 taken into consideration. Side by side with 

 the practical requirements the scientific 

 phase of the subject has been kept in view, 

 with full faith in the belief, which is based 

 on the history of science, that the things 

 which to-day are speculative and abstruse 

 will to-morrow belong to the commonplace 

 applications of science to the daily wants 

 of the community. The survey now main- 

 tains a small magnetic observatory in Porto 

 Rico ; a complete and modern one at Chel- 

 tenham, ilaryland; another at Baldwin, 

 Kansas; one at Sitka, Alaska; and yet 

 another near Honolulu in the Hawaiian 

 Islands. We may hope, therefore, that the 

 United States will contribute no small 

 share towards finding the mysterious cause 

 of the earth's magnetism, or at least in 

 fni-nishiiii;' the data necessary for a more 



