"I am glad to hear you discuss the subject, my lads," he remarked in a kind voice.  "Though you are wrong in your conjectures, if you will attend, I will try and explain what I know about the matter.  It is a very important one, for by means of this dust--for dust it is--which fills the air, philosophers have been able to determine in part the difficult problem of the track of the winds in their circuits.  How is this? you will say.  Dust coming from one place surely cannot be distinguishable from dust coming from another.  To the ignorant man it is not, but to the man of science it is.  There are certain minute animal productions called infusoria and organisms peculiar to each portion of the globe.  The expression is, the habitat of such infusoria is such or such a place.  These infusoria can only be distinguished by a most powerful microscope.  Professor Ehrenberg, who has devoted his attention to the subject, has examined specimens of the dust which is now falling on our decks.  He found it composed of dry infusoria, the forms of which are found not on an African desert, but in the south-east trade-wind regions of South America."