GENERAL NAVIGATION. 9 



cies. United States consulates, and foreign hydrographic offices. 

 Copies are furnished free by the main office or by any of the branch 

 offices on application. 



With each notice is sent also a separate sheet, giving the items re- 

 lating to lights contained in the latest Notice, intended especially for 

 use in correcting the Light Lists. 



Pilot Charts of the North Atlantic and North Pacific oceans are 

 published near the beginning of each month. These charts give the 

 average conditions of wind and weather, barometer, percentage of 

 fog and gales, and routes for steam and sailing vessels for the 

 month of issue; ice and derelicts of the preceding month; ocean 

 currents and magnetic variation for the year; storm tracks of pre- 

 ceding years; and much other useful information. They are fur- 

 nished free only in exchange for marine data or observations,* 



Hydrographic Bulletins, published weekly, are supplemental 

 to the North Atlantic Pilot Chart and contain the latest news of 

 wrecks and derelicts along the American coast and ocean routes, 

 Arctic ice, reports of the use of oil to calm the sea, and other infor- 

 mation for mariners. They are to be had free upon application. 



THE USE OF CHARTS. 



Accuracy of chart. — The value of a chart must manifestly de- 

 pend upon the character and accuracy of the survey on which it is 

 based, and the larger the scale of the chart the more important do 

 these become. 



To judge of a survey, its source and date, which are generally 

 given in the title, are a good guide. Besides the changes that may 

 have taken place since the date of the survey in waters where sand or 

 mud prevails, the earlier surveys were mostly made under circum- 

 stances that precluded great accuracy of detail ; until a plan founded 

 on such a survey is tested it should be regarded with caution. It may 

 indeed be said that, except in well- frequented harbors and their ap- 

 proaches, no surveys yet made have been so minute in their exam- 

 ination of the bottom as to make it certain that all dangers have been 

 found. The fullness or scantiness of the soundings is another method 

 of estimating the completeness of the survey, remembering, however, 

 that the chart is not expected to show all soundings that were 



* Pilot charts of the South Atlantic Ocean are to be published for each of 

 the four seasons of the year, beginning with the quarter which comprises the 

 months of June, July, and August, 1909. In their properties and aims and their 

 mode of construction and issue they will be similar in all respects to the 

 Pilot Charts of the North Atlantic and North Pacific oceans. Pilot Charts 

 of the South Pacific and Indian oceans are also marked out for regular pub- 

 lication. 



