over a Mosler Safe, the "Rock of Ages" that survived the blast. Space is nearer.
The advertisement is for a brand of safe (Mosler) whose motto is, "rock of ages," meaning it can survive anything, even the blast of Hiroshima—which, in the name of commerce, is truly tasteless: they're using an image of human devastation to sell their safes.
"Space is nearer" echoes the previous line, "The ditch is nearer," and it seems to be part of a bad-vibes series of statements. Like, things are getting worse.
In this case, the phrase could mean a couple of things: 1. That our advances in technology (evidenced by the bomb, the safe, and although not mentioned, the recent successful human exploration into space) literally gets us closer to discovering outer space. 2. It might carry a deeper, more figurative meaning: that space (as in nothingness) is nearer, because we're destroying our history by failing to commemorate it or simply forgetting it, and we're destroying our potential future by tearing up our cities with reckless abandon to make way for what? Er, more parking? Things are looking bleak.
When I crouch to my television set, the drained faces of N**** school-children rise like balloons.
The speaker is describing the African American school children he sees on TV. Keep in mind that this poem was written during the time of the Civil Rights movement.
He describes the faces as "drained." Considering what the poem has so far been about—the members of 54th black regiment sacrificing themselves for the Republic—their faces might be drained from decades of sacrifice and struggle. We know from history that African Americans continued the struggle for human rights long after slavery was abolished and long after the Civil War, into the poem's present Civil Rights struggle. Perhaps Lowell is trying to illustrate how slow-going and difficult progress has been for African Americans in this country.
He contrasts the negative word, "drained" with a happier "balloons." So even though the faces are drained, they are still rising. The line ends on a potentially more positive note.