Why does Jim grab the light cord at the last minute?

Jim grabs the cord in order to pull himself up high enough to see through the cabin window of the ship.

Why I should have done so I can hardly say. It was at first mere instinct, but once I had it in my hands and found it fast, curiosity began to get the upper hand, and I determined I should have one look through the cabin window.

I pulled in hand over hand on the cord, and when I judged myself near enough, rose at infinite risk to about half my height and thus commanded the roof and a slice of the interior of the cabin.