x.] DISEASE. 165 



well understood cause of such clouds is the deflection 

 of a wind laden with invisible vapour, by means of 

 the sloping flanks of the mountain, up to a level at 

 which the atmosphere is much colder and rarer than 

 below. Part of the invisible vapour with which the 

 wind was charged, becomes thereby condensed into the 

 minute particles of water of which clouds are formed. 

 After a while the process is reversed. The particles 

 of cloud having been carried by the wind across the 

 plateau, are swept down the other side of it again to a 

 lower level, and during their descent they return into 

 invisible vapour. Both in the cloud and in the 

 population, there is on the one hand a continual supply 

 and inrush of new individuals from the unseen ; they 

 remain a while as visible objects, and then disappear. 

 The cloud and the population are composed of elements 

 that resemble each other in the brevity of their exist- 

 ence, while the general features of the cloud and of the 

 population are alike in that they abide. 



Preliminary Problem. — The proportion of the 

 population that dies at each age, is well known, and the 

 diseases of which they die are also well known, but the 

 statistics of hereditary disease are as yet for the most 

 part contradictory and untrustworthy. 



It is most desirable as a preliminary to more minute 

 inquiries, that the causes of death of a large number of 

 persons should be traced during two successive genera- 

 tions in somewhat the same broad way that Stature 

 and several other peculiarities were traced in the pre- 



