306 Mr Wilsoji, On the Formation of Cloud. [May 13, 



Such a top displayed the characteristic properties of a celt, 

 and a deviation of 15' displayed itself in the motion: with a 

 deviation of 6° each direction of rotation was unstable for either 

 longitudinal or transverse oscillations and an angular velocity 

 imparted was reversed three times before motion ceased. Several 

 other peculiarities of the top were likewise pointed out. 



(5) On the Formation of Cloud in the Absence of Bust. By 

 C. T. R. Wilson, B.A., Sidney Sussex College. 



The cloud-formation is brought about as in the experiments 

 of Aitken and others by the sudden expansion of saturated air. 

 A form of apparatus is used in which a very sudden and perfectly 

 definite increase in volume is produced, and in which all danger of 

 the entrance of dust from the outside is avoided. If we start with 

 ordinary air, after a small number of expansions to remove dust 

 particles by causing condensation to take place upon them, it is 

 found that the expansion has now to be pushed to a certain definite 

 limit in order that condensation may take place. With expansion 

 greater than this critical amount (working with a constant initial 

 temperature) there is invariably a cloud produced, and none with 

 less expansion. 



Some preliminary experiments have given the following results. 



— = 1"258, when initial temperature = 16°*7C. 



Here — is the ratio of the final to the initial volume, when 



?i 

 condensation just takes place. 



This corresponds to a fall of temperature of about 26°C., and to 

 a vapour pressure about 4'5 times the saturation pressure. 



In order that water drops should be in equilibrium with this 

 degree of supersaturation their radii must be equal to about 

 8'3 X 10~^ cm., assuming the surface tension for such small drops 

 to have its ordinary value. 



