52 Scientific Proceedings, Royal Dublin Society. 



our experiment is, that the water-column being continuous, but air 

 being present, a suction of less than one atmosphere can still 

 operate as a suction more than 12 m. lower down." 1 



In order to obtain a more definite idea from these general 

 statements, we will apply the figures obtained by Copeland in 

 his experiment. The rise in the upper manometer was 550 mm. 

 This subtracted from the atmospheric pressure — say, 760 mm. — 

 gives us the effective pressure exerted by the atmosphere at the 

 level of 8*4 m. It is 210 mm. of mercury. In the same way the 

 effective pressure below is 760 mm. - 428 mm., or 332 mm. Accord- 

 ing to Copeland, then, a column of water, 8*4 m. high, which is 

 equivalent to 617 mm. of mercury, is lifted and supported by a 

 pressure equal to the difference between 332 mm. and 210 mm., or 

 122 mm. of mercury. Such an overbalancing or equilibrating of 

 a greater force by a lesser may, of course, readily be shown to 

 lead to perpetual motion. 



Suppose a glass-tube, having the same dimensions as Cope- 

 land's, and filled with water, were erected beside, and connected 

 above and below with, the tube filled with plaster of Paris. The 

 column of water in the clear tube being 12 m. high will exert a 

 pressure on the water in the bottom of the plaster of Paris, at 

 least amounting to one atmosphere. This pressure, according to 

 Copeland' s deduction, is more than sufficient to raise the water to 

 the top of the plaster of Paris ; and so it will flow over, across the 

 upper connexion, and maintain the clear tube full of water. In 

 this way a continuous circulation would be established. 



The observations and deductions which have led to this 

 untenable position are due principally to the neglect of two 

 important factors, which may be, in the first instance, briefly 

 summarized as follows : — 



1. The readings of the manometers are not a true measure of 

 the pressure-conditions of the water in the plaster of Paris, but 

 rather of local differences of vapour and air-pressure. 



2. Plaster of Paris continues to absorb water for a long time 

 after it has set. This absorption, by reducing the volume of water 



1 Zee. cit., p. 166. 



