The poet intends to portray the stream's subtle and nearly unnoticeable flow by utilizing the phrases "steal" and "slip." These words together with the image of a stream that "steals" or "slips" past its surroundings describe how the water is able to move without being noticed. This idea is reinforced by the use of the word "silent" just before the phrase "stream steals". In addition, by describing the brook as "black" and "rapid" the poet wants to emphasize its mysterious nature. Although it is a familiar sight, we tend to take the brook for granted because of this. If someone stole into your backyard at night and began to flow rapidly across the ground while black smoke rose up from where it touched the earth, you would feel threatened even if you knew it was only a brook. Finally, the poet uses the word "graceful" to describe the movement of the brook. It is not an aggressive or violent motion; rather, it is quiet and smooth. In conclusion, the poet wants to convey the brook's stealthy quality by using the words "steal" and "slide" in his poem.