How does Swift make himself appear to be an expert? (ethical appeal)
Swift could be very critical and contemptuous of self-proclaimed experts, and this is often reflected in his satire. The Royal Society in London regularly published papers by scientists putting forward grand proposals that they claimed would solve many of society's ills. The proceedings of the Society are famously satirized by Swift in Gulliver's Travels when professors of the Grand Academy of Lagado carry out pointless experiments such as attempting to extract sunbeams from cucumbers. And "A Modest Proposal" is written in much the same spirit.
As it is expressly designed to parody the kind of scientific papers published by the Royal Society, the "Proposal" is written in a suitably learned prose. However, as it is also an appeal to the audience's emotions it presents itself as being an ethical treatise concerned primarily with tackling the twin scourges of poverty and famine. By a constant appeal to the emotions, Swift is cleverly diverting our attention away from the revolting details of the proposal towards its alleged moral benefits. And of those there are many. A higher value will be placed on children, as they will become precious commodities; there will be fewer Catholics in Ireland, thereby contributing to the stability of the Protestant state; poorer tenants will finally have something of value in the children that they breed, instead of the corn and cattle that is usually seized by their landlords anyway.
Swift's "Modest Proposal" prefigures the work of utilitarians such as Jeremy Bentham, who believed that society should pursue the greatest good of the greatest number. In other words, what mattered was not the intrinsic morality of a particular act, but its effects in generating overall happiness. Satire or not, "A Modest Proposal" has more than a touch of the utilitarian about it.
Swift is a master of satire...and ethos.
He begin by establishing himself as a man who has spent years studying the problem, and offering evidence that he has "maturely weighed" the various schemes of others who have offered solutions, and found them "grossly mistaken in the computation."
His plan also proposes to "prevent those voluntary abortions" so many of these poor women have, and he points out that we can all agree that murder of one's own child is horrible. Thus, he notes and agrees with the values of his audience, establishing further ethos. Along these lines, he points out that the children of the poor quickly become a scourge on society, being forced to turn to begging and stealing themselves, which is another thing no one wishes.
At the end of the essay, he points out that he has nothing to gain himself, clarifying that his proposal is for the good of the people of Ireland only:
I have not the least personal interest in endeavoring to promote this necessary work, having no other motive than the public good of my country, by advancing our trade, providing for infants, relieving the poor, and giving some pleasure to the rich. I have no children by which I can propose to get a single penny; the youngest being nine years old, and my wife past child-bearing.