Among other things, the work introduced the concept of cells into the biological sciences, set the stage for the use of fossils as a record of past life and investigated the nature of combustion. It also proposed an unbuilt design for a device for grinding lenses of large diameter and long focal length. The final sections of the work deal with astronomical observations and a calculation of the height of the atmosphere.
The Great Fire of London (Sunday, 2 September —Thursday, 6 September 1666) improved Hooke's financial situation considerably. Less than a week after the fire, he produced a model for the rebuilding of the devastated section of the city, and while his plan was not adopted, Hooke became one of three official surveyors for the reconstruction, which took decades.
As, Hooke played a significant role in establishing empirical approaches to the understanding of nature and science in general. He anticipated the invention of the steam-engine, formulated Hooke’s Law of the extension and compression of elastic bodies (a spring's extension or displacement from its neutral position is directly proportional to the force applied), and developed the balance-spring in watches, and the anchor-escapement for clocks. He anticipated Isaac Newton's law of the inverse square in gravitation (1678), constructed a telescope with which he made significant discoveries and materially invented the microscope, the quadrant, and several of instruments associated with meteorology. The latter included a range of barometers, rain gauges, hygroscopes (measuring humidity), a wind speed instrument and a weather-clock to record readings from a number of different instruments automatically.
Sources: Chambers Biographical Dictionary; Patri J. Pugliese, Robert Hooke (1635–1703): Wikipedia