The first paragraph ends at "they began gradually to come down to us." What it intends to convey is the magnetism of Finny's personality. He is a natural leader who draws people to him. At this juncture, he is dismayed with the idea of badminton as a summer sport at Devon.
Imagery—description using any of the five senses of sight, sound, touch, taste, and smell—that convey the idea of Finny's magnetism are as follows:
The first image is a simile. Finny is likened to the weather: "Phineas was always being watched, like the weather." This shows how important Finny is to the other boys, and how they are attuned to him and adjust to him, just as they would the weather. He is a basic part of their life at the school. Second, continuing this metaphor, Finny is compared to the wind: his leaving is like a "shift in the wind." We all know what it is like to both be aware of the weather--how the sky darkens or lightens, how the wind can get harder--so we can understand the effect Finny has on the other students. Finally, the repeated alliterative "w" sounds in both sentences have a whooshing sound like the wind that reinforces how Finny's presence or absence directs people towards him.
In the second paragraph, Finny's leadership is again the point. When people have clustered around him, he speaks, showing his dominance. Imagery comes up in the second sentence, in which Finny looks first at Gene, his second-in-command, and then at the others—we can visualize this happening. Second, because of the alliteration, the emphasis falls on the image of "dazed determination" on Finny's face, showing that, in this case, he wants to impose his will on the others and change the game from badminton to something else. Finally, he blinks twice, something we can also visualize, and ends by directing everyone's attention to the heavy ball. At this point, because of Finny's winning personality, it is clear a game with the ball will ensure.
In both paragraphs, imagery and alliteration establish Finny as a central figure to the other boys.