The deal will make the full texts of 250,000 books from the British Library available to researchers online.

A treatise on the hippopotamus and a description of the first engine-driven submarine are among 250,000 books to be made available online in a deal between Google and the British Library.

The agreement, announced June 20, will let internet users read, search, download, and copy thousands of texts published between 1700 and 1870.

It is a small step toward the library’s goal of making the bulk of its 14 million books and 1 million periodicals available in digital form by 2020.

“So far we have only been able to digitize quite a small fraction of the global collection,” said the library’s chief executive, Lynne Brindley. “There is a long way to go.”

The deal with Google, which will see 40 million pages digitized over the next three years, will offer online researchers a selection of rarely seen works from an era of social, political, scientific, and technological change that spanned the Enlightenment, the Industrial Revolution, and the American war of independence.

The books range from Georges Louis Leclerc’s “Natural History of the Hippopotamus, or River-Horse” to the 1858 work “A Scheme for Underwater Seafaring,” describing the first combustion engine-driven submarine.

The books are more than scholarly curiosities. British Library curator Kristian Jensen said an 18th-century guide to the English language for Danish mariners shows “how English began to emerge from being the language spoken by people over there on that island” to become the world’s dominant tongue.

Google will pay to digitize the books, which are no longer covered by copyright restrictions. They will be available on the British Library and Google Books websites.

Peter Barron, Google’s European spokesman, declined to say how much the project would cost, beyond describing it as “a substantial sum.”

Google has digitized 13 million books in similar deals with more than 40 libraries around the world. But its plan to put millions of copyrighted titles online has been opposed by the publishing industry and is the subject of a legal battle in the United States.

Barron said the company’s goal “is to make as wide a range of items as possible” available online.

“Having richer content means people around the world are searching more for it, and that is good for our business,” he said.

Last year, the British Library announced plans to digitize up to 40 million pages of newspapers dating back three-and-a-half centuries, and it recently made thousands of 19th-century books digitized in a deal with Microsoft available as an app for iPhone and iPad devices.