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78 ground and can be seen from some distance, so that it is not long before some wanderer finds it; and he also finds a label attached to it, saying that if he will send the apparatus and records to the Meteorological Office he will get five shillings reward. I am sure five shillings would be very useful to many of you, so I advise you to keep a sharp look-out whenever you are in the country to see whether you can find a "spider," and get the five shillings reward.

There is one thing about the upper atmosphere which these sounding-balloons do not tell us, though if they could bring down some of it they might; and perhaps it may be arranged at some future time that they shall bring samples down with them, and then we should be able to verify what we believe to be true, viz., that certain gases, which are present in our lowest layer in very minute quantities, become much more common up there. When I was a boy, we were taught that the air consisted of oxygen, nitrogen, carbon dioxide (or carbonic acid gas, if you like that name better), and water vapour; nothing else but these four. But in the year 1894 Lord Rayleigh announced to the British Association, which was then meeting at Oxford, that he had found something else in the air—a gas so like nitrogen that he had had the utmost difficulty in separating the two, but clearly different from nitrogen when sufficient care was taken to divide them. It was a most exciting announcement, because we all thought that we knew practically all there was to be known about the nature of air, and it was a great shock to find that we were all wrong. The new gas was called argon; and soon other new gases were found, especially by