Page:Physical Geography of the Sea and its Meteorology.djvu/361

Rh chances are capable; and farther, these results are so certain that there is no longer any room for the mariner to be in doubt as to the best route. When a navigator undertakes a voyage now, he does it with the lights of experience to guide him. The winds and the weather daily encountered by hundreds who have sailed on the same voyage before him, with "the distance made good" by each one from day to day, have been tabulated in a work called Sailing Directions, and they are so arranged that he may daily see how much he is ahead of time, or how far he is behind time; nay, his path has been literally blazed through the winds for him on the sea; mile-posts have been set up on the waves, and finger-boards planted, and time-tables furnished for the trackless waste, by which the ship-master, even on his first voyage to any port, may know as well as the most experienced trader whether he be in the right road or no.

623. Close running.—From New York to the usual crossing of the equator on the route to Rio, the distance, by an air line, is about 3400 miles; but the winds and currents are such as to force the Rio bound vessel out of this direct line. Nevertheless, they have been mapped down, studied, and discussed so thoroughly that we may compute with remarkable precision the detour that vessels attempting this route from New York, or any other port, would have to make. This computation shows that, instead of 3400 miles, the actual distance to be accomplished through the water by vessels under canvas on this part of the voyage is 4093 miles. More than a hundred sailing vessels have tried it by measuring and recording the distance actually sailed from day to day; their mean distance is 4099 miles, consequently their actual average differs only six miles from the computed average.

624. A desideratum on ship-hoard.—The best navigated steamships do not sail closer than this, and a better proof of the accuracy of our knowledge concerning the prevailing direction of the winds at sea could not be afforded. Unfortunately, anemometers are not used on shipboard. Had they been in common use there, and had we been furnished with data for determining the force of the wind as well as its direction, we could compute the time as well as the distance required for the accomplishment of any given voyage under canvas. Thus the average time required to sail from New York to the equator might be