In my description of soaring flight I have used the term “soarable air,” implying thereby that in air in which birds can soar some factor is present whose nature is not yet understood. That the use of this term is advisable and necessary will, I think, be admitted by anyone who will read the following account of some remarkable phenomena observed at the stern of a steamer during a dust storm in the Red Sea.

In the first place it must be premised that gulls frequently find conditions suitable for soaring flight anywhere at a height of a few hundred metres above sea level, not only in tropical seas but also off the south coast of England. On the other hand, with rare exceptions, gulls are only able to soar near sea level in a curiously restricted area on the leeward side of the stern of a steamer. The passage of the steamer has caused some change in the air in virtue of which air otherwise appearing as “unsoarable” now behaves as “soarable air.”

It is a commonplace of laboratory experience that one or two months' practice may be required to “get one's eye in” for an apparently so simple task as taking the readings of a polarimeter. But it does not seem to be so widely realised that similar practice is necessary to get one's eye in for seeing objects that are only visible for minute periods of time. Consequently, anyone who has had such practice is liable to incur criticism from those who do not realise its nature and value.

Colonel de Villamil is sceptical as to my statement that in certain cases the smoke issuing from a steamer funnel assumes the form of two vortex filaments lying side by side and rotating in opposite directions. The changes of shape of the smoke masses and their rotary movements are so rapid that they are by no means easy to recognise. I have seen the movements in question both on P. and O. steamers and on excursion steamers from Bournemouth. Mr. J. H. Field, of the Agra Aerological Observatory, kindly permits me to state that he has frequently seen the double spiral in steamer smoke as I have described it. His observations were ,made independently of mine and before he had seen my paper. The necessary condition for the formation of the spirals is that a strong wind should be blowing across the top of the funnel.