Gunpowder like all other inflammable substances requires to be raised to a certain temperature, before it will ignite, viz., to a dull red heat, or about 600° Fahrenheit. If the heat passes with such rapidity through the powder, so as not to raise the temperature to the necessary degree, then the powder will not ignite, from the velocity of transit, so that it might be possible to calculate theoretically, the velocity that must be given to a red hot ball to enable it to pass through a barrel of gunpowder without causing explosion. The passage of electric fluid through gunpowder may be adduced in evidence of the ignition being dependent on the degree of velocity. The flame of all fulminating powders will pass through the centre of a box filled with gunpowder without igniting one grain of it. If a train of gunpowder be crossed at right angles by a train of fulminating mercury, laid on a sheet of paper or a table, and the powder be lighted with a red hot iron wire, the flame will run on until it meets the cross train of fulminating mercury, when the inflammation of the latter will be so instantaneous as to cut off all connection with the continuous train of powder, leaving the remaining portion of the gunpowder unignited. If on the contrary the fulminating powder be lighted first, it will go straight on and pass through the train of gunpowder so rapidly, as not to inflame it at all. Were a gun to be charged with gun-cotton and gunpowder, the latter would be fired out unignited. EFFECTS OF ACCIDENTAL EXPLOSIONS OF GUNPOWDER. Considering the combustible nature of the materials, accidents very seldom occur; when they do, it is more frequently in the process at the Mill while under the runners. On one occasion at Waltham Abbey Mills, when the powder exploded, after having been two hours under the runners, the doors and windows of the Mills on the opposite side of the stream, were forced open outwards, and the nails drawn. A similar effect took place when the Dartford Mills blew up, January 1833, in consequence of an accident in the packing house. A window which had been recently fitted up in Dartford Town, about a mile and a half distant from the works, was blown outwards into the street, and a considerable quantity of paper was carried as far as Eltham and Lewisham, distances of eight and ten miles. The sudden rarification of the air may account for this circumstance, the atmospheric pressure being removed in the vicinity of the doors and windows, they were forced open outwards by the expansive force of the air contained within the buildings. ON ANCIENT ENGINES OF WAR. ~War a painful topic.~ ~Advantages of war being destructive.~