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29.

In this tank, the gases were shot down the straight pipe, entered the upper U-pipe at a small angle, thus avoiding any considerable rebound, and thence passed around the circular part--not returning to the straight pipe until the velocity had been greatly reduced by friction. 

In order to make the time, during which the velocity was being reduced, as long as possible, the pipes were carefully cleaned of scale. They were first pickeled, and then cleaned by drawing through them, a number of times; first, a scraper of sheet iron, second, a stiff cylindrical bristle brush, and finally a cloth. All but the most firmly adhering scale was therefore removed. Further, care was taken to cut the hole in the rubber washers, between the flanges, so wide that compression by the flanges would not spread the rubber into the pipe, and thereby obstruct the flow of gas. 

Notwithstanding all these precautions, evidence was had that the gases become stopped very rapidly. This was to be expected inasmuch as there is solid matter; namely, the wadding and wire, that is ejected with the gas, which accumulates with each successive shot. This solid matter must offer considerable frictional resistance to motion along the U-pipe, and, since the mass of gas is only of the order of a gram, must necessarily act to stop the flow in a very short time. This interval of time was great enough, however, so that this second method afforded a satisfactory check upon the first method. 

A possible modification of the above two methods would have been to provide some sort of trap-door arrangement whereby the gases, after having been reduced in speed in a container as just described, would have been prevented from returning upward into