Situation of Zones without Bain, and of Deserts. 363 



the summer season, the reverse naturally taking place in the 

 other hemisphere. 



The preceding results can be rendered more palpable by 

 the assistance of curves, in order to draw which, it is only ne- 

 cessary to raise on an equatorial line equidistant perpendicu- 

 lars representing the different months. Those which represent 

 the months of spring, of summer, and of autumn, fill the in- 

 termediate space, in such a manner that the summer occupies 

 the middle. This first sketch will be completed by the dif- 

 ferent latitudes ; and by co-ordinating the pluviometrical in- 

 dications by means of these axes, we shall have for our hemi- 

 sphere, first of all, on the equatorial line, the rectilineal 

 band of nearly perpetual rains ; there will then come a curve, 

 whose two branches, starting from points at the equator, re- 

 presented by the months of spring and of autumn, will con- 

 verge towards the tropic in the months of summer, and all the 

 interior of this arch being shaded, will express the semestrial 

 rains degenerating into trimestrial rains towards the summit 

 of the curve. Lastly, concentrically with the preceding, there 

 will be a second curve, whose branches having their origin to 

 the north of the tropic, at points represented by the winter 

 months in the latitudes of Algeria, will afterwards pass by 

 points represented by the months of spring and of autumn in 

 the latitudes of the south of Europe, and will become, in the 

 parallels which are rainy during the summer months, a per- 

 pendicular to the equator, prolonged from the latitude of Paris 

 to the North Pole. 



This mode of expressing succinctly the results of observa- 

 tion, of course does not take into consideration local disturb- 

 ances, upon which I do not mean to insist at present, as they 

 form the subject of a very extensive separate investigation, 

 with which I have been occupied for several years. It pos- 

 sesses, however, the advantage of exhibiting a very remark- 

 able phenomenon — that, namely, of the abrupt transition 

 which presents itself from the zones of the trimestrial rains 

 of the tropics, to that of the trimestrial rains of the opposite 

 seasons of the sub-tropical zone of Von Buch. Now, a sudden 

 leap of this kind not being in harmony with the ordinary 



