But even putting aside the high altitude idea, and confining our route-tracing to the known courses of air currents, we shall find immense difficulty in mapping out the actual course of any bird on any particular day The study of some of the publications of the Meteorological Committee, such, for instance, as the "Life History of Surface Air Currents," by Shaw and Lempfert, published in 1906, shows the great variation in the pathways, speeds, and formation of these systems; a bird which accidentally entered a cyclone would unconsciously alter its actual track and speed very many times before it passed beyond the area of influence.
I am indebted to Mr Stubbs and Mr Herbert Taylor of King's College, London, for some interesting mathematically worked—out routes of birds, travelling at a given speed in a cyclone rotating at given speeds and moving at a fixed rate; these show great variation both in direction and speed according to the time and place of entering the system. The track of the bird is, of course, influenced by its own rate of progress, by the speed of the rotating currents, and by the rate at which the whole system moves in any direction. Thus a migrant passing south and earning within the influence of a cyclone which is moving north-east at a high rate of speed, say 40 miles per hour, will, if it enters towards the northern limits of the system, be all first retarded by the con-
