air can impinge on the gauze. The shield had the disadvantage of not allowing the collier or the deputy to check that the gauze was in place and clean. Lamps were therefore made so that they could be inspected and then the bonnet placed on and locked. Timeline of development Examples of lamps Davy lamp In the Davy lamp a standard oil lamp is surrounded by fine wire gauze, the top being closed by a double layer of gauze. If firedamp is drawn into the flame it will burn more brightly and if the proportions are correct may even detonate. The flame on reaching the gauze fails to pass through and so the mine atmosphere is not ignited. However, if the flame is allowed to play on the gauze for a significant period, then it will heat up, sometimes as far as red heat. At this point it is effective, but in a dangerous state. Any further increase in temperature to white heat will ignite the external atmosphere. A sudden draught will case a localised hot spot and the flame will pass through. At a draught of between 4 and 6 feet per second the lamp becomes unsafe. At Wallsend in 1818 lamps were burning red hot (indicating significant firedamp). A boy (Thomas Elliott) was employed to carry hot lamps to the fresh air and bring cool lamps back. For some reason he stumbled; the gauze was damaged and the damaged lamp triggered the explosion. At Trimdon Grange (1882) a roof fall caused a sudden blast of air and the flame passed through the gauze with fatal results (69 killed). Poor copies and ill-advised "improvements" were known, but changing dimensions either reduced the illumination or the safety. The poor light compared to either the Geordie or Clanny eventually led to the Davy being regarded as "not a lamp but a scientific instrument for detecting the presence of firedamp". Some pits continued to use candles for illumination, relying on the Davy to warn men when to extinguish them. Stephenson ("Geordie") lamp In the earlier Geordie lamps an oil lamp is surrounded by glass. The top of the glass has a perforated copper cap with a gauze screen above that. The glass is surrounded by a perforated metal tube to protect it. Air ingress was via a series of tubes at the base. Later versions used a metal gauze, instead of the