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.”

A mathematical investigation of the small oscillations of a kite was proposed in “Stability in Aviation” in the form of a problem (p. 180, problem 16). A solution of this problem has been given by Prof. J. M. Bose in the Bulletin of the Calcutta Mathematical Society, Vol. II., No. I. Unfortunately his investigation contains many serious errors of a fundamental character, the effect of which is to render the solution inapplicable to a system in any way resembling an ordinary kite. As examples, the variations in the components of the tension of the kite string are assumed to depend on the velocity components instead of on the displacements of the kite, although Prof. Bose's previous equations show the contrary to be the case. Again, while in his introduction he considers the case of a kite attached by a forked string he contradicts himself by neglecting the displacements of the point of intersection of the string with the plane of the kite.

Some years ago, during my stay in Rio de Janeiro, I busied myself during spare time in determining the value of the normal pressure (by means of a plane mounted on a rotary arm) in such a way that no disturbing induced air current could arise.

For this purpose I had at my disposal a room 12 metres square by 8 metres high. I employed an apparatus by which a rotating arm 9 metres long described a quadrant or quarter circle. The test surface was 0.25 square metres in size. Since the surface traversed the quarter circle only once it encountered only still air. Uniform motion was controlled by the curve described by a vibrating spring on the end of which was fastened a small paint brush with colour. The spring was fastened one meter from the turning centre and vibrated over a paper spread on a measuring table. (Figures 1 and 2, not reproduced.)

The results were however not reliable enough on account of the shortness of the observations and the small size of the room. After my return to Germany, in the spring of 1913, I therefore undertook new experiments on a larger scale. Through the kindness of the director of the South–West Electricity Works, there was placed at my disposal a large empty machinery hall in Schöneberg of 32 metres clear width, 60 metres long and 15 metres high.

This paper is intended as a sequel to “Stability in Aviation” and deals with* Problem 2 in Chapter XI. (p. 178). The long list of problems there given will show that before the book was written or even commenced, investigations had been projected and contemplated more than sufficient to fill a second volume on the Rigid Dynamics of Aeroplane Motions, my book on stability constituting the fiist. It will be found that the necessity of mathematical investigation in connection with stability, and other aeroplane problems was first mentioned by me in “Science Progress” for October, 1897, and it would have been a great advantageif the completion of most of this work had preceded the first aeroplane flights. With reasonable facilities and assistance, longitudinal stability could have been completed by about ,1900, lateral by 1904; the unwritten second volume would have been in the printers’ hands by January, 1910.

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.