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Tests have been carried out on the effect of a number of factors on the venting of explosions in a straight unobstructed length of 1 ft duct. It has been found that the efficiency of a given venting area for a given length of duct may be increased by a factor of about ten by distributing the venting area over the whole length of the duct. There was little difference if a vent of a given area was placed in the end of the duct or near the end in the side of the duct. Variation of the weight of the cover of the vent between 0 and 720 g/ft2 of venting area gave rise to a variation in maximum pressure of about 50 per cent. The particular point in the pressure record where the maximum pressure occurred varied considerably between the tests. This suggested that the maximum pressure was controlled by one of the following factors: (1) the inertia of gas in the duct, (2) the inertia of the cover of the vent, (3) the increase in combustion rate produced by turbulence at the vent itself, (4) the pressure drop across the vent.