This is certainly one of the excellent aspects of this essay. It is important to think about how the tone that Swift creates fits into his satire as a whole. Swift very cleverly creates a tone that increases the impact of his barbaric and shocking "modest proposal," which heightens the satire and irony of the piece. Note how, before proposing his solution to the Irish famine, the speaker makes every effort to present himself as a caring, sensitive and earnest individual who sincerely wishes to find a solution to this problem. The repeated reference to statistics likewise shows how credible the speaker is, as he has obviously done his research well. Thus the suggestion of rearing Irish babies as a food source is all the more shocking.
However, what is essential to realise about this excellent essay, is the way in which there are two separate tones that are operating side by side. Although the "surface" tone is reasonable and earnest, the deeper tone is one of bitterness and sarcasm. The sheer preposterous nature of the "modest proposal" is evidence of this deeper tone. Swift is very angry about the British government's inability to do anything about the famine, and this anger and resentment seethes beneath the respectable surface of this essay.