And what country can preserve its liberties if their rulers are not warned from time to time that their people preserve the spirit of resistance? Let them take arms. The remedy is to set them right as to facts, pardon and pacify them. What signify a few lives lost in a century or two? The tree of liberty must be refreshed from time to time with the blood of patriots and tyrants.
Sunday, March 16, 2014
Vortex rings
Vortex rings are wonderful at maintaining coherent vorticity while moving over significant distances. If you stand several meters from a foam cup and try blowing to knock it over, it’s not likely to budge. But move the air impulsively with a vortex cannon, and you can knock it over from the opposite side of the room. The same principle works underwater with added visual effect. Here an impulsive burst of air exhaled by the diver forms a bubble ring with vorticity strong enough to knock over a stack of rocks. It may look like a superpower, but this is science! Dolphinsand whales are also known to play with this trick. For the non-scuba-divers among you, it’s also possible to learn to do it in a swimming pool.
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