in 1690, halley built a diving bell, a device in which the atmosphere was replenished by way of weighted barrels of air sent down from the surface. in a demonstration, halley and five companions dived to 60 feet in the river thames, and remained there for over an hour and a half. halley's bell was of little use for practical salvage work, as it was very heavy, but he made improvements to it over time, later extending his underwater exposure time to over 4 hours.