482 REPORT—1904. 
During the last few years much has been done in the construction of navigable 
balloons, but only proving, in my opinion, how insuperable are the difficulties of 
attaining really practical results. The aeroplane, which may be defined as a plane 
surface propelled through the air at a small horizontal inclination, so that the 
resulting pressure of the air supports it against the action of gravity, gives promise 
of far better results. Wings, vertical-acting screws, and aeroplanes proper all 
come under this definition. The action of the air on an inclined surface requires 
more study, as theory and practice in this matter are at considerable variance. 
With small models, as shown, a vertically lifting screw can, despite theory, 
lift as great a weight into the air asa horizontally propelled plane. I have been 
making a number of experiments in this line, as well as with man-carrying 
aeroplanes driven by the impetus gained by sliding down an inclined plane. The 
latter haye been chiefly to get at an idea of the strength required in the different 
parts of construction, and to test the balance of the machine. But results in all 
these lines tend to show that a great amount of careful experiment is necessary 
before we can hope for good results, though the prospects are decidedly hopeful. 
7. Plato’s Theory of the Planets. 
By Professor D’Arcy W. Tuompson, C.B. 
8. Report on Underground Temperature.—See Reports, p. 51. 
9. Zur Flugfrage. By Dr. F. Hirret, 
10. Upper Air-currents and their Relation to the Audibility of Sound. 
By Rev. J. M. Bacon, 
Investigations carried out during a long series of balloon ascents have revealed 
a very remarkable complexity in the upper air-currents which, from their nature, 
would escape the notice of the observer on earth. A number of light bodies, of 
varying sizes and differently constituted, have been prepared and allowed to float 
away into space at different heights and under different circumstances ; and these, 
carefully watched, have shown the existence of minor but headlong currents, hold- 
ing determined courses frequently at variance with that of the balloon, It has 
been proved that dominant but diverse air-streams will glide one above another in 
juxtaposition without commingling, and that upper currents maintaining the same 
level will occasionally alter their course, presumably in obedience to some con- 
figuration of the earth below; while, at all heights, ascending or descending air- 
streams, greater or lesser, will obtrude themselves in a way which is often wholly 
unaccountable, 
Ina manner equally capricious, and apparently dependent on the above, sounds 
conveyed through the upper air will be carried sometimes to abnormal distances in 
directions at variance with the ground current, being borne to earth over far but 
favoured plots of ground, while they may pass unheard over districts which might 
be considered well within sound range. These results, which have been obtained 
largely by organised observation of the hearing of aerial bombs, will presumably 
account for the occasional surprisingly far travel of sound signals; or, again, their 
failure at short ranges. 
1l. On the Effect of Electric Avr-currents. 
By Professor Srtim Lemstr6m. 
The author, after referring to suggested explanations of the magnetic and electrical 
conditions of the earth, draws attention to the fact established by Wijkander that 
