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Rh together, and there are people who can make a fascinating noise in that way. I have here two beautiful planes, that Mr. Whitworth told us how to make; they fit one another so closely that we can exclude the air without a pump. We must not merely put them together in the ordinary way; we must slide one on to the other in order to squeeze out the air: and then you find them stick so that they are as hard to pull apart as the Magdeburg hemispheres.

These two experiments show the enormous pressure air has. But that is only near the Earth; as we go up it gets less and less; or, perhaps, a better way to look at it is that as we come down in the air the pressure gets greater, because every layer has to support all the layers above it. Fig. 18 is a picture of a totem-post in the Oxford Museum, kindly made for me by a member of the audience and her mother. It used to be outside the tent of an Indian chief. There is a raven at the bottom, and on that a bear holding a hunter; then another bird; above that another bear hugging