The pressure of steam permitted in a steam locomotive boiler above which the safety valves will blow off the excess. A good crew will attempt to work the locomotive without reaching this pressure as operation of the safety valves wastes steam. Early boiler pressures were low. Stephenson's 'Locomotion' of 1825 had a pressure of 25 pounds per square inch (psi), while his 'Rocket' of 1829 had a pressure of 50 psi. Ten years later, boilers with 100 psi capability were being built. By the turn of the century pressures had reached 200 psi on some larger locomotives and rose to 300 psi on some of the largest US locomotives. 280 psi was the highest pressure used in the UK.