What is the literal meaning of the metaphor "For it is as the air, invulnerable" found in Hamlet?

In Act I, Scene 1 of Shakespeare's Hamlet, 
the guard watches the shores of Denmark as there is an fearful apprehension
that Norway's Fortinbras, son of the late king who was slain by King Hamlet, who took
his lands, plans an attack in order to retrieve such lands.  This guard has seen during
their watches a spectre; however, Horatio says that Marcellus and Bernardo and Francisco
are merely imagining it.  So, Marcellus has entreated Horatio come and watch with
them.

The ghost, in fact, does appear, looking like the
dead King Hamlet.  Horatio orders it to speak, but the ghost disappears into the night;
Horatio believes that the ghost of the king "bodes some strange eruption to our state"
(1.1.70).  Again the spectre appears and Horatio urges it to speak to him, telling if it
knows anything of Denmark's fate.  As the ghost starts to move away, Horatio calls to
Marcellus, telling him to stop it:

readability="8">

Shall I strike at it with my
partisan?

Do, if it will not stand.
(1.1.139-140)

But, Bernardo
and Horatio find themselves stabbing at the air. Marcellus
concludes,

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'Tis gone.

We do it
wrong, being so majestical

To offer it the show of
violence;

For it is as the air,
invulnerable,

And our vain blows malicious
mockery (1.1.143-145)

By
stabbing at the ghost of King Hamlet, Bernardo and Horatio make a mockery of killing the
majestical king, for he is already dead and, like the air that they have stabbed
invulnerable, or untouchable and incapable of being wounded or killed since it is
already dead.  The phrase quoted above in the question is precisely a simile as
mentioned in the previous post since the comparison is stated using as
(which means like).