Two funny novels make for good reading in the hammock; Bridge of Birds, by Barry Hughart. New York: St. Martin's Press. 248 pp. $13. 95. The Complete Knowledge of Sally Fry, by Sylvia Murphy. New York: G. P. Putnam's Sons. 172 pp. $13.95.

Call them beach books or porch books or hammock books, or whatever: Here are two first novels that provide perfect summer reading. ''Bridge of Birds'' is subtitled ''A Novel of an Ancient China That Never Was ,'' but those readers who don't like fantasies and prefer stories with their feet on the ground shouldn't be scared off. This is a very funny book, and in Number Ten Ox we have a narrator whose ingenious outlook lends reality to the most fantastic of adventures.

Ox and Li Kao are seeking the ginseng Great Root of Power in the hope that it will save the poisoned children of Ox's village, who lie in a mysterious coma. Their search leads them to a heartless adversary and takes them into, among other places, a labyrinthine cave from which they escape with a Butch Cassidy-and-the-Sundance-Kid leap, and a salt desert from which they escape by building the Bamboo Dragonfly, a helicopter.

''I am beginning to suspect,'' remarks Li Kao, a sage with a self-admitted slight flaw in his character, ''that the simple quest for a ginseng root is wrapped in more riddles than that Mysterious Mountain Cavern of Winds, where the White Serpent crushes heroes in the cold coils of enigmas; and while I am probably hallucinating, I am willing to bet that the ghost of a murdered maiden fits in here somewhere.''

In contrast to the murder and mayhem described with wicked zest, this page-turner of a novel also contains touching love stories and some very beautiful nature writing.

Another new first novel, ''The Complete Knowledge of Sally Fry,'' is a contest-winning English comic work written in dictionary-encyclopedia form.

It had earlier struck Sally that writing one's own encyclopedia could be ''a useful form of self-realization therapy.'' Unfortunately, she told her idea to Bruce, her lover, and ''he wrote a thesis about it that earned him his doctorate and his director's post at You-Know-Where. I had no one to blame but myself. How green was my Sally.''

Bruce's theft comes under various headings, including FAIR, as in unfair. Under KNIFE comes his assumption that she herself is incapable of completing a doctorate.

So we meet Sally at her wealthy mother's seaside cottage in Cornwall, planning to spend a quiet summer working on her thesis but distracted by worries about her illegitimate son, a disturbed young man who has recently disappeared. Thus she begins writing her ''Complete Knowledge'' in order ''to try to come to terms with my problems.''

And then peace explodes into a hilarious uproar, as family members descend on the cottage with their own problems. There are five nieces and nephews, two sisters with marital difficulties, and their husbands - not to mention the retired commander (''well laced with gin'') living next door with his young wife who has a penchant for cricket and caves.

This novel is in the tradition of madcap English comedies, and comparisons with many other writers, such as Margery Sharp, spring to mind. But Sylvia Murphy has a voice of her own, ironic and witty. Great fun!