This line gives us more detail about how the jar is influencing the wilderness.
We know that the jar is on a hill, and we can imagine that the crest of that hill is bare and grassy.
The wilderness—trees, branches, vines, birds, skunks, rabbits, yadda yadda—around the hill is being described as rising up to the hill and the jar.
Hmm. We're presented with a question of a pronoun here—namely, "it." "It" can be taken as either the hill itself, or the jar on top of the hill.
If we consider "it" to be the hill, we might imagine that, literally, if you look at an unkempt hill, you might see its groundcover and trees as moving upward to the top, as if it were actually rising. The hill is making the wilderness rise (since, you know, it's a hill and therefore higher than its surroundings).
On another level, though, "it" can also be the jar that sits on top of the hill. Woah. This brings in a more abstract option: if you've ever heard the expression "rise to the occasion," you'll read this line as not only describing a rise in altitude, but a rise in character.
It's interesting that the jar is considered a high point for the wilderness to rise up to. As a man-made object seen as having control over its natural surroundings, it makes us question whether the jar (the human influence) is seen as empowered over nature. Or, perhaps the wilderness is seen as on the same level, equal in importance, as this mundane human invention.
And sprawled around, no longer wild.
Not only does the wilderness seem to be drawn to this jar like some kind of magnet, but the jar has made the wilderness no longer wild.
Yet, the wilderness still sprawls, or spreads out. We think that this seems kind of wild. Men would neatly manicure and order the wilderness if it were no longer wild, yet it sprawls out, like a person who has dropped down in the middle of a long walk to take a nap in the soft grass of a field. In that way, the wilderness is treated as human through further personification.
We have, however, a determined statement that, under the rule of the jar, the wilderness is no longer wild. Possibly, this is because the jar has intruded.
There's something man-made in the wilderness now, tarnishing its purity. It could also be a statement about how men, and man-made objects, often overtake the wild and the natural. We're not sure, however, whether the speaker thinks this is a good thing or a bad thing. Maybe the lines to come will shed a little light for us.
The jar was round upon the ground
We return from the description of how the wilderness interacts with the jar to a description of the jar itself. The description of the jar, which is simply round and on the ground, makes it all the more amazing that the wilderness has (seemingly) been conquered, or at least affected, by it.
Note that this line is basically a repetition of line 2. We are hearing, for the second time, that it, like most jars, is round. Instead of hearing that it is on a hill, this time, we find out that it's on the ground.
Also, note the contrast of sound in this line. We've got "round" and "ground," echoing the "around" from the line before. Yet the line starts with the word "jar." Even though "round" is supposed to describe the jar, "jar" is quite a different sound, and not really a round one at all. The J jumps out at us, forcing us to use our mouth in a weird way that stands out from the rest of the poem. (For more on this effect, check out the "Sound Check" section.)
So, though this jar commands the wilderness—at least taming it so that it's no longer wild, possibly rounding out its rough edges—this rounding out itself is, like the jar, out of place, jarring.
And tall and of a port in air.
We get more description of the jar in this line, though it's not as straightforward as the description in the rest of the poem.
The jar is tall—which probably means it's just a little longer than other jars, not that it's seven feet tall like a basketball player. (Though if it were, it would probably be called Kareem Abdul-The-Jar. Get it? Any fans of the 1980s Lakers out there? No? Okay, moving on…)
Then we've got the whole "of a port in air," which is a little difficult to get into—to say the least. The word "of" can stand in for the word "like," which would make this phrase a simile.
But then we have to consider the different meanings of the word "port." When we first think of the word "port," we think first of a harbor—the place where ships come in and out, delivering people and goods across water. But port can really be any entry place, where people or things come in and out. Port can also be used to describe the way that someone carries him or herself. It also makes us think of the world "portly," which means heavy, or large.
If we think of the jar as a port in air, as in a harbor or entryway, we can still think of it in two ways (very tricky, Mr. Stevens). One way would be to think of the air itself as the ships that come in and out of a typical port. Certainly, in the wilderness outdoors, air would come in and out of the jar.
We can also think of the air being like water for a normal port. Instead of being in the water, this jar is a place where, in the air, commerce can happen and things can be exchanged. In this way, perhaps most importantly, it's a way to access a new place or world.
If we think of port as in "portly" or heavy, though, we've already got the basis to think of it as round. Round and tall, the jar has a heaviness that shows that it, although being inanimate, bears itself with pride.
We think that the line combines these few meanings. The jar is a port for the commercial world (after all, it's the commercial product of manufacturing) to enter the wilderness and possibly vice versa. It's also quite round and heavy, unlike the wilderness, which we connect with barely scraping together its survival—flourishing, but thin.
To sum up this tricky bit of analysis: however you read it, the jar is again in sharp contrast with the wilderness that surrounds it.