Christopher Wren imagined and designed a weather-clock which could register the traces of multiple meteorological instruments (barometer, thermometer, rain gauge, hygrometer etc.). Wren presented his machine on different occasions, under different forms: verbal and written descriptions, sketches and illustrations, and maybe even as a physical object. Being charged with building Wren’s weather-clock, Robert Hooke took fifteen years to present the results of his work to the Royal Society. In the end, Wren and Hooke’s machine was never used in practice. In this paper I argue that the stubborn attempts of several inventors to build a weather-clock were driven by the special nature of this invention which seemed to be within one's grasp. Because the functions and operations of the machine could be graphically imagined and described, one easily formed the impression that such a machine could also be built in practice.