THE DIMENSIONS OF TRAVEL 



and breadth have meaning only as they are relative to motion. Thus the 

 fourth dimension of travel is the velocity of the traveler within the space of 

 the other three coordinates : time in the space of travel, or time-in-space. 



Velocity of travel is commonly measured against two kinds of interval, 

 one of distance, the other of time. The term miles per hour gives an idea 

 of the relation between distance and time, but does not relate to the height 

 and breadth of travel. If we are to study animal movement in a truly objec- 

 tive manner, we must consider the whole structure of its four dimensions. 



Let us now examine a specific problem in bird migration. Evidence sug- 

 gests that the fall passage of Canvasbacks from Delta to Lake Christina is 

 made in one flight, a trip of about 300 miles. Canvasbacks have been ob- 

 served on this course at an elevation of 2,000 feet and at a measured ground- 

 speed of 60 m.p.h. At this height the breadth of travel is about 118 miles, 

 or 59 miles on either side of the line of flight. If a tail wind prevails through- 

 out, the time-in-space of this journey is 5 hours. The dimensions of such a 

 flight are thus: length, 300 miles; time-in-space, 5 hours; height, 2,000 feet; 

 breadth, 118 miles. This equation for a Canvasback is compared with the 

 travel coordinates of a man who walks at 4 m.p.h., * at an eye elevation of 

 5% feet, and a breadth of travel (in the absence of obstacles) of about 6 

 miles. Time-in-space, 5 hours, is the common denominator for drawing com- 

 parisons, with all other coordinates relative to this. With the time constant 

 for man and bird, the relative value of the man's distance is 20 ; that is, 20 

 miles is to man as 300 miles is to Canvasback. We of course recognize this 

 relativity in our daily movements. As I walk over the open prairie to the 

 marsh a mile away, I see a duck flying in the same direction. The bird's 

 absolute mile is the same as mine, but its relative distance is much shorter. 



The foregoing considered only two coordinates: distance and time-in- 

 space. These are relative to the height of travel, 2,000 feet for Canvasback, 

 5)a feet for man. Bear in mind that height is not an isolated point in space ; 

 it flows with the traveler's motion and is perceived by the eye as movement 

 of the earth itself, an optical illusion in which the ground beneath the 

 traveler seems to approach from ahead, pass underneath and to either side, 

 then recede behind. The velocity of this apparent ground flow is relative to 

 the height of motion. In human experience, for example, the ground seems 

 to speed rapidly past in a blur when our aircraft leaves the runway, the 

 velocity of this ground flow decreasing as the aircraft gains altitude. Air 



• Four miles per hour is not an average walking speed for man, but an experienced 

 walker in top condition can average this on an overland journey. 



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