Manchester Memoirs, Vol. xliv. ( 1 899), No. o. 1 1 



examined not ox\\\ the influence of obliquity, but also of 

 tlie shape and presentation of the plane. His results for 

 the case to which (5) relates indicate an even greater 

 relative effect at small angles, probably referable to the 

 back suction. A laboratory experiment to demonstrate 

 the reality of this suction was described in one of the 

 papers already referred to (^Nature, loc. cit.). 



Experiments upon the law of obliquity, as executed 

 for the case of air, b}' Dines* and Langley,f involve cum- 

 brous and costly whirling machines, and if made in the 

 open are greatly embarrassed by wind. An apparatus 

 capable of working in the laboratory^ or as a lecture 

 illustration, has long been a desideratum. With the aid of 

 Mr. Gordon I have recently constructed one which, while 

 ver}' simple and inexpensive, performs sufficiently well. 

 It may be regarded as a kind of adjustable windmill. 

 An axis of hard steel, finely pointed at the ends, is 

 carried by agate cups. From a central boss six spokes 

 of round steel project symmetrically, carrying at their 

 ends six similar vanes of tin-plate. The vanes are pro- 

 vided with projecting sockets of brass tubing, which fit 

 the spokes somewhat tightly, but yet allow the vanes to 

 be rotated when desired. The vanes are 4 inches long 

 and I?, inches wide, the distance of their inner ends from 

 the axis being about 37 inches. The whole apparatus 

 is as light as may be (about 120 gm.) consistenth^ with 

 the necessary rigidity. 



If the vanes are all inclined at the same angle, the 

 apparatus works like an ordinary windmill, and may be 

 set into rapid rotation by a motion through the air parallel 

 to the axis. This motion may take place either in a 

 horizontal or in a vertical direction. If means were 



* Proc. Koy. Soc, June, 1S90. 



+ Siitiihsoniau Coiitrilnitioiis to Kiw'vledge, 1 891. 



