Royal Institution of Great Britain, 169 



calculated to show the effect, are given, and the true theory stated. 

 The effect in question is easily shown by making a small hole 

 through a flat smooth bung or piece of wood, inserting a tube 

 formed of a quill, so that it shall not project on the smooth side 

 beyond the surface ; then sticking three pins perpendicularly into 

 the cork or wood, at about three-quarters of an inch distance from 

 the quill hole, for the purpose of loosely confining a round disc of 

 card or paper, which is to be laid over the hole. In this state of 

 things, if the mouth be applied to the end of the quill, and an 

 endeavour be made to blow off" the disc, it will seem as if the latter 

 were urged in opposition to, and pressed agaimt the current of 

 air ; adhering consequently to the surface of the cork or wood, and, 

 by covering the aperture, tending to stop the hole, and prevent the 

 passage of the air. 



The cause of the effect exists within the space between the two 

 flat surfaces, and was said to depend upon the momentum commu- 

 nicated to the particles of air, which tended to make them move 

 with equal velocity from the centre of the space between the two 

 discs, to that part corresponding with the circumference in the direc- 

 tion of radii ; but as the air, under these circumstances, is conti- 

 nually passing from a smaller to a larger space, the tendency to 

 preserve its velocity must cause a partial vacuum, so that, except 

 in a direction opposed to the course of the radiating current, the 

 pressure, or rather resistance, of the air between the two discs in 

 these parts, is less than the pressure of the atmosphere. Just at the 

 centre of the disc, the force of the current of air passing down the 

 tube is added to the elastic force of the air there, and the two toge- 

 ther are greater than the pressure of the atmosphere on the oppo- 

 site side of the same part of the disc : but as the disc is governed 

 as a whole, not by the forces upon any one part, but by the means of 

 the forces acting upon its two faces, so it will move from or towards 

 the fixed surface, according as the mean of the forces exerted upon 

 its inner face is greater or less than the uniform pressure of the 

 atmosphere upon its outer surface. Now, although at the centre 

 of the disc the force acting perpendicularly upon the inner surface 

 is greater than the pressure of the atmosphere ; that excess is more 

 than compensated for by the diminution (dependent upon the 

 momentum of the air, as already described) of the forces acting in a 

 similar perpendicular direction upon the more extensive parts to- 

 wards the circumference ; and the mean of all the powers is found 

 to be less than the pressure of the atmosphere, consequently the 

 latter has the predominance, and the disc is urged against th^ 



