During the medieval period mechanised bellows were introduced to blow air into the furnace, thus increasing the speed and efficiency of the smelting process.   The power to operate these bellows was provided by water wheels and buildings were constructed to house the apparatus.   These structures are known as “blowing houses” and on Dartmoor twenty-three examples have been found.   In Cornwall only five surviving examples are known to the writer and these were at St. Neot, Hurdon [SX 210823], Trekeive [SX 228701), Godolphin [SW 603320) and Retallack.   However, from documentation, it is clear that there were once very many more sites, and the details of these are shown in Table IV and Figure 3.14.   The reasons for a shortage of surviving examples in Cornwall are numerous.   Firstly, many have been converted into grist mills because their large size and existing water supplies made them ideal choices, at a time when blowing houses were being replaced by reverbatory smelting works and the increasing Cornish population demanded a greater corn grinding capacity (Barton, D.B., 1971, 69).   Secondly, during the blowing process tin may have escaped into the ground, walls and roof of the house, and thus, when a site became redundant it could have been worthwhile to demolish it to obtain this lost metal.   This factor would have affected the survival of blowing houses throughout the south-west, but it is possible that the custom was more prevalent in Cornwall.   Thirdly, the mould stones, which are so characteristic of this type of site are easily transported and have often been removed and reused as animal feed troughs. Finally, Cornish blowing houses were situated closer to the centres of population than those on Dartmoor, with the result that many of the redundant examples were probably robbed for building stone.