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 very large part of the North American continent. The so-called heat-wave then seems to have travelled eastward, and crossed the Atlantic Ocean, but, owing to the absence of information, except in such casual records as may be found by an inspection of ships' logs, we know little or nothing of the actual progress of the heated region across the Atlantic. However it may have come about, it is, at all events, certain that a fortnight after the occurrence of unusually great heat in the New World there was a similar experience in the Old World. Our knowledge of the distribution of temperatures over the whole globe is too incomplete to enable us to follow the movements of the great wave as fully as we might desire. No doubt our own Meteorological Office does most admirable work, and of course many other countries have more or less complete organisations for the study of meteorological phenomena, yet our information as to the thermal condition of the globe still falls short of what we would like to have. Certain materials are, however, available, and we shall endeavour to throw what light we can on the matter.

We often hear the question asked as to what was the cause of this exceptional heat. Let me hasten to say that neither in these pages nor anywhere else could I attempt to answer this question in the sense in which it has usually been proposed. It is very doubtful whether it would be possible to assign a single cause for such a phenomenon, even if we knew many things of which we are now completely ignorant. Indeed the most difficult problem of astronomy becomes simplicity itself when compared with the extraordinarily complex agents that are in operation even in the simplest meteorological phenomenon. Let me illustrate this contrast between the two sciences by an example.