Page:A Voyage in Space (1913).djvu/179

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not follow the track calculated for it, and ultimately two very clever men, Adams, an Englishman, and Leverrier, a Frenchman, calculated that there must be a yet undiscovered planet pulling it out of the place; they independently calculated whereabouts this unknown planet must be, and in that position it was actually found.

Fig. 44.

One thing I would like you to realize is the very small indication they had to go upon. You know stories of scouts who have been able to track their prey by small indications, a bent twig here, a footprint there—indications which our untrained eyes would pass unnoticed? Well, the indications left by Uranus in his journey were small and faint like those, so that clever scouts were required to read them properly. Suppose Uranus travelling in the circular path at the top of Fig. 44; let us put the real Uranus and the Uranus-as-it-might-be (undisturbed) both in their places; then you could not see the difference between them at all. More than that: suppose you magnify that circle on the screen 50 times, so that it becomes a big circle nearly the size of the room. We shall only have space on the screen for a little bit of it, which looks almost straight, owing to the