.Hark! do you hear the sea? Edgar, still in 'disguise' as Poor Tom, is claiming that he has brought the blind Duke of Gloucester, his father, to the edge of Dover cliff. But there is a wider point worth considering here - to do with language here and in the play as a whole.  Edgar will create through words alone the impression that they are truly at the cliff's edge.  He is using language to create something that is not - ie a lie. The    nature of the relation between Language and Truth seems to be a theme running through the play as a whole.  Consider Cordelia on the one hand and Goneril and Regan on the other in Act 1, scene 1. There, Cordelia's goodness is defined in terms of her honesty - her inability to say any more than the simple truth: ' I love your majesty/ According to my bond; no more nor less.' ,in contrast to Goneril and Regan's fluency with language's ability to speak in excess of what actually is the case: ' Sir, I love you more than word can wield the matter; / Dearer than eye-sight, space and liberty...'.  Cordelia approaches, almost, a rejection of language because it can be used to lie: 'What shall Cordelia speak?  Love, and be silent.'  Think also of Kent and his plain honesty ('Sir, 'tis my occupation to be plain...Act 2, scene 2, 88 - although Kent when he says this is actually in disguise, his words continue to name what is what.)  

Edgar, the corresponding figure to Cordelia in the sub-plot, quite unlike Cordelia, has a bewildering array of voices - Poor Tom, the ' better spoken' figure who is speaking here, the man on the beach who picks up Gloucester after his 'jump', the rustic who will kill Oswald, the knight with a 'lost name' who will challenge Edmund. Cordelia always is what she is:  Edgar uses deception, 'untruth', but for benign purposes.  As he says in this scene: 

                Why I do trifle thus with his despair

                Is done to cure it.

(Is there any relation of these points to the last speech of the play:  

                 'The weight of this sad time we must obey;

                  Speak what we feel, not what we ought to say...'   ?)

Why, then your other senses grow imperfect..(perhaps relate to Lear's remarks in Act 3, scene 4, lines 6-12:'...but where the greater malady is fix'd,/The lesser is scarce felt.'- that is, how one feeling is driven out by another.  There is perhaps a running theme through King Lear of how characters/human beings' attention is subject to becoming dominated by one thing - think of Lear taking his eye off the state by his own more personal concerns (retirement, love), or the more evil characters' narrow focus on their own greeds, despite moral and practical consequences.  There is perhaps a wider question here - the fluidity of feeling in King Lear how feeling is subject to alteration, so that Edgar, despite earlier words, realises that there is always the possibility of feeling worse:'And worse I may be yet; the worst is not/So long as we can say'This is the worst.' (Act 4 scene 1, 27-28) )

..in nothing am I chang'd/ But in my garments.. Edgar has to say this, doesn't he, to preserve his disguise and preserve his project to cure Gloucester.  But Gloucester has drawn attention to something true:  as Edgar's 'project' proceeds, his language does alter - becoming gradually more his own, marked by the return of his lines to verse.  It is as though the project isn't simply to rescue Gloucester from his despair, but also to gradually recover Edgar's own identity.  

Secondly, Edgar's words are true, in an interesting way:  whatever his disguises of language and appearance, his essential self remains untouched - he has, unchanged, the same sympathies and moral reactions (see his frequent responses in his asides to the King's and his father's misfortunes).  But that isn't true of Goneril, Regan and Edmund - for all of these three, the discovery that language can be cut loose from what actually is true is the beginning of a progressive moral decline: they go on getting worse and worse, as though their characters have nothing to fix them.  They go from ordinary human beings, with some justice on their side perhaps, into freefall, ending up 'monsters'.  It is as though deception is a dangerous acid, corrosive of character, and only to be handled safely by good characters like Edgar and Kent.

..Here's the place..Edgar creates through words alone the sights, the feeling, of being on the cliff's edge. The speech has a peculiar effect; at the immediate level, Edgar has to persuade his father that he really is at the edge of Dover cliff - and he does - but the speech in doing so widens the view of the play outwards to momentarily take in a view of the ordinary world outside the miseries of the main characters.  (In this widening of perspective it is perhaps like Lear's 'naked wretches' prayer in Act 3 scene 4, lines 28-36, or, in its sense of placing personal tragedy against the background of the wider world, and so reaching a kind of perspective on it, it perhaps looks forward to W H Auden's Musee des Beaux Arts.)

The best critical writing on this speech to my mind is in John Bayley's Shakespeare and Tragedy:

'The workaday intimacy in the little exchange between the pair now bursts exuberantly out.  In his mindless emphases (' horrible....fearful....dreadful trade....')Edgar apes a cheerful working-man's relish in the way things are.  In the midst of tragedy we briefly glimpse the daily round of hazard and accident - fires, floods, falling off ladders....The crows and choughs, the mice-like fisherman, the samphire gatherer, are beheld by the spectators as if they had abruptly floated off into a world outside the play...'

..the murmuring surge...How does this fit with Edgar's earlier question: ..do you hear the sea?  It emphasises how, in Bayley's words, 'it is a sea heard in the mind only..'

..for all beneath the moon/Would I not leap upright.  An 18th century editor wanted to change Shakespeare's word 'upright' to 'outward' - after all, you would land back in exactly the same spot if you were to jump upright.  There's a nice put-down of this critic by Dr Johnson - to the effect that if the critic was realyy to try to prove this on a cliff's edge, the world would be one critic less.  The substantial point is the precision of Shakespeare's words.

Why I do trifle thus with his despair / Is done to cure it.  Why does Edgar delay revealing that it is his son who is leading him?  Especially as his father will shortly say:  If Edgar live, O, bless him!  and Edgar has his own doubbts about the effects of his course of treatment:  And yet I know not how conceit may rob / The treasury of life when life itself / Yields to the theft..'  This present aside of Edgar's shows Shakespeare's awareness of the question forming in the audience's mind - and is an attempt to set it aside.  Shakespeare wants the peculiar spectacle of Gloucester's 'mock' suicide - and wants the audience to focus on that, not consider Edgar's tactics.  Edgar's aside is a reassurance, before we get on with the business in hand.  (it might be possible to defend Edgar's policy with Gloucester as a lesson in learning patience - that rich word in King Lear, and as attempting to re-enchant Gloucester's sense of being alive - Thy life's a miracle.  See later notes.)