I was wondering: what single thing did the most to advance humankind’s lot? Speech must score pretty highly, I suppose, as must writing, paper, discovering how to light a bonfire and Google image ...

Bores are sealed with ‘O’ rings' sited in grooves, (figure 1). The ‘O’ rings' are usually of toroidal shape (i.e. circular cross section) and are regulated by BS 1806 and BS 4518. Figure 1. ‘O’ Ring ...