Travel BY AIR

Whereas there are expressions like to travel by boat/ bus/ car/ plane, which contain the name of a vehicle or method of travel, there are also those like to travel by air/ land/ sea, which have the name of space or area in which vehicles travel. This picture reminded me of these two kinds of expressions.

This was at a cafe, where I stayed for several hours chatting with two of my close friends over a cup of coffee.