The large propeller used in these tests was built without special regard to weight, since it was expected that it would be subjected to rather rough usage under the very sudden strains produced by the irregular working of the gas engine. Its hub was made of brass tubing, the horns being brazed to rings which were slid over a central tube, the rings being finally soldered to the tube after the arms had been adjusted to the positions which would give the blade the correct shape and dimensions. The wooden arms were 1.5 inches in [p180] diameter at the hub end, tapering to 1.25 inches at the end of the blade. The blade was exceedingly stiff as regards pressure produced by thrust, but it was found to be considerably strengthened and made very much safer when guy-wires were added, in the manner explained above. This general type of construction was adhered to in all the future propellers for the aerodrome, though slight modifications, both as to the size of the arms and the number and position of the cross-pieces which formed the framing of the blade, were adopted from time to time. A pair of heavy propellers, 2.5 metre, 1.25-pitch ratio, 36-degree blade, the hubs of which were formed of brass castings, was, however, constructed for experimental purposes, where weight was not an important factor. When these propellers were designed, the calculations as to their size and the horse-power which would be required to drive them at a certain speed were based on the very incomplete data obtained from the various propeller tests conducted during the preceding years. When later calculations were made for them, on the data obtained in the more accurate tests made in the summer of 1898, it was found that the power of the engines with which it was proposed to equip the aerodrome would not be sufficient to drive the propellers at anything like the speed which the former calculations had shown would be possible; and that, therefore, either the ratio of the gearing between the propellers and the engine would have to be changed so as to permit the engine to run at a very much higher speed than the propellers, or that propellers, having either less pitch or a smaller diameter, and possibly both, would have to be substituted for these larger ones.