Tristram asks the reader to "imagine to yourself" Dr. Slop, a "little, squat, uncourtly figure of a Doctor Slop, of about four feet and a half perpendicular height," knocked off his little pony into the mud by Obadiah, who comes charging around the corner of the stable on his big coach horse.
Walter and Toby are astounded to see him, the former measuring in his mind the very short time elapsed since he rang the bell to send off Obadiah. The same phenomenon instantly brings into Toby's mind "Stevinus, the great engineer." Tristram promises to explain why, "but not in the next chapter."
Writing is like conversation, says Tristram: you leave your partner something to imagine. Imagine, then, he says, Dr. Slop all cleaned up and changed. But Dr. Slop has left his bag of obstetrical instruments at home.
Obadiah is dispatched again to get them.
Toby tells Dr. Slop that his "sudden and unexpected arrival" reminded him of the great Stevinus. Walter bets that Stevinus is a writer on fortifications, and so he is. Toby lectures Dr. Slop about military terminology until Walter expostulates with him about his lack of feeling for Mrs. Shandy's "pains of labour." "I wish the whole science of fortification, with all its inventors, at the devil."
Toby does not take offense; apropos of his gentleness, Tristram observes that "my uncle Toby had scarce a heart to retaliate upon a fly," and he tells the story of Uncle Toby and the fly. Once, he caught an "overgrown" fly that had "tormented him cruelly all dinner-time," and he put it out the window saying, "Go poor devil, get thee gone, why should I hurt thee? — This world surely is wide enough to hold both thee and me." Tristram learned "one half of [his] philanthropy" from that event.
Toby's good nature moves Walter to beg his pardon, and the brothers are tenderly reconciled. Walter, by the way, announces that the conception of children doesn't give him much pleasure.
Toby picks up the subject of Stevinus again; that famous engineer had built a wind-propelled "sailing chariot" that traveled on land. Dr. Slop's improbably-speedy appearance seemed to be due to such a contrivance. Dr. Slop has himself seen the machine in Holland, and he points out its superiority over horses, "which . . . both cost and eat a great deal." Walter begins his theory of economics but is interrupted by the opening of the door.
