A VOYAGE IN SPACE
though larger. There is one hung up near the roof; perhaps we can get it down at the end of the lecture. These balloons do not carry men, they only carry apparatus; but this apparatus is so skilfully made that it brings down for us information about these upper layers. We learn that the pressure gets less and less, as we expected; but we also learn something that we did not at all expect. We thought that it would get colder and colder as we went higher, but these "sounding-balloons" (ballons sondes) tell us that soon after we leave the first layer it ceases to get any colder. The figures for the temperature in Fig. 20 show that it does not get very much colder. They are measured from what we call absolute zero. You know well the ordinary thermometer which your mother puts into your mouth when she thinks you "have a temperature." If we used that we should have to alter these figures a good deal: and if your mother used the one the Meteorological Office uses, I fear she would get a great fright, for she always hopes to find your temperature below 100°, doesn't she? Yet with this thermometer she would find it over 300°, even if there was nothing the matter with you at all. But, of course, if she were warned beforehand that that was the proper temperature for you to have with this kind of thermometer, she would not be alarmed: there are such various kinds of thermometers, that before using any of them it is well to know beforehand what to expect.
Well, the great fact we have recently learnt about these upper layers of the air is that the temperature ceases to fall in a way that has been a great surprise