Sir

As the original patentee of the nuclear radiometer concept, I am impressed with Daedalus's imaginative applications of the idea (Nature 392, 337; 1998 & Nature 392, 443; 443; 1998).

The basic patent (Self-Propelled Nuclear Radiometer, US Patent No. 3,110,811) almost 40 years ago employed precisely the principle that Daedalus describes — to create thrust for small motors or in outer space. Five years earlier, that principle was employed in another patent (Nuclear Radiometer for Neutron Flux Measurement, US Patent No. 2,951,942), using a neutron absorber such as fissionable materials or boron-10, to create particles to heat one side of radiometer panes to measure neutron flux.