When two sheets of fabric, stitched on frames to represent the two skins of a rigid airship, were hit by a grenade of the naval type with a four-ounce charge, it was found that the front sheet was blown to shreds and the rear sheet had a hole about half a foot in diameter blown in it. Later experiments at Farnborough against balloons filled with hydrogen, and made to resemble as nearly as possible a section of a rigid airship, were completely successful. Firing at floating targets, and at small target balloons released from the aeroplanes, was practised at Eastchurch. It was found that, with no burst or splash to indicate where the shot hit, this practice was unprofitable. The effective use of small-bore fire-arms against aircraft was made possible by two inventions, produced under the stress of the war itself, that is to say, of the tracer bullet, which leaves behind it in the air a visible track of its flight, and of the incendiary bullet, which sets fire to anything inflammable that it hits. At the outbreak of war the only effective weapon for attacking the Zeppelin from the air was the Hales grenade. Of two hundred of these which had been manufactured for the use of the Naval Wing many had been used in experiment; the remainder were hastily distributed by Lieutenant Clark Hall among the seaplane stations on the East Coast. The Naval Air Service experimented also with the mounting of machine-guns on aeroplanes. On this matter Lieutenant Clark Hall, early in 1914, reported as follows: 'Machine-gun aeroplanes are (or will be) required to drive off enemy machines approaching our ports with the intention of obtaining information or attacking with bombs our magazines, oil tanks, or dockyards.... I do not think the present state of foreign seaplanes for attack or scouting over our home ports is such as to make the question extremely urgent, but I would strongly advocate having by the end of 1914 at each of our home ports and important bases at least two aeroplanes mounting machine-guns for the sole purpose of beating off or destroying attacking or scouting enemy aeroplanes.'