Page:My Airships.djvu/102

 the rate of 20 miles an hour. Imagine the current to be 10 miles per hour. If he navigates against the current he accomplishes 10 miles an hour with respect to the shore, though he has been travelling at the rate of 20 miles an hour through the water. If he goes with the current he accomplishes 30 miles an hour with respect to the shore, though he has not been going any faster through the water. This is one of the reasons why it is so difficult to estimate the speed of an air-ship. It is also the reason why air-ship captains will always prefer to navigate for their own pleasure in calm weather, and, when they find an air current against them, will steer obliquely upward or downward to get out of it. Birds do the same thing. The sailing yachtsman whistles for a fair breeze, without which he can do nothing, but the river steamboat captain will always hug the shore to avoid the freshet, and will time his descent of the river by the outgoing, rather than the incoming, tide. We air-shipmen are steamboat captains and not sailing yachtsmen. The navigator of the air, however, has the one great advantage—he can leave one current for another. The air is full of varying currents. Mounting, he will find an advantageous breeze