162 OBSERVATIONS ON THE FALL OF RAIN. 
years, are necessary, and in districts of various 
physical and geological character. 
The quantity of water which may be expected 
to flow from a flat country well clothed with vege- 
tation, will be very different from that which will 
pour in torrents down the steep declivities of 
uncovered mountains ; and again, the water which 
will pass off from the surface of a limestone dis- 
trict, with its many caverns and perpendicular 
fissures, will vary considerably from that which 
will drain off in like manner, from a district, 
similar perhaps in its external features, but con- 
sisting of the dense rock of a primitive formation 
or the flat beds of the coal measures.—Clay and 
sand; morass and cultivated ground; absorbent 
chalk and more impervious material, all contribute 
to produce a varying result. 
The rapidity with which rain descends, or the 
state of saturation in which the ground is found 
at the commencement of a wet period, will also 
vary the quantity of water which will flow away 
in different years as a supply to rivers, although 
the average or gross amount of rain may be nearly 
the same. 
