Here are the first three sentences in Jeans' first paragraph. Notice what information Jeans includes next and how it relates to what he wrote in the first two sentences:

Because of the way it came into existence, the solar system has only one-way traffic, like Piccadilly Circus. The traffic nearest the center moves fastest; that further out more slowly, while that at the extreme edge merely crawls, at least by comparison with the fast traffic near the center. It is true that even the furthest and slowest of the planets covers nearly three miles every second, which is about 200 times the speed of an express train, but this is a mere crawl in astronomy.

Jeans has told you that just as the traffic in a roundable moves at different speeds, so do the planets. In the third sentence, he picks up on this idea and explains in specific terms how this is so and gives an example of the speeds with a familiar object, the express train.

Now, look at these three sentences. What element that Jeans has already discussed do you think he will pick up in his fourth sentence? When you think you know, click on NEXT. Here are the first three sentences again:

Because of the way it came into existence, the solar system has only one-way traffic, like Piccadilly Circus. The traffic nearest the center moves fastest; that further out more slowly, while that at the extreme edge merely crawls, at least by comparison with the fast traffic near the center. It is true that even the furthest and slowest of the planets covers nearly three miles every second, which is about 200 times the speed of an express train, but this is a mere crawl in astronomy. NEXT