56 
METEOROLOGY OF 1 897 
throughout the year. The shallow temperatures, even 
when smoothed by using the weekly averages, show much 
greater variations than the deeper ones. Some time is 
taken for the surface temperature to affect the lower ther- 
mometers. Practically the daily variation disappears be- 
fore reaching a depth of six feet, and the daily change does 
not often exceed one or two-tenths of a degree. The heat 
wave moves slowly through the soil, so that while the high- 
est temperature of the day is reached at a depth of three 
feet at nearly the same hour as at the surface, its tempera- 
tures correspond to those of the day before. This cannot 
be seen in the present diagram. The curves do, however, 
especially in the winter, show the lagging of the annual 
curve of temperature. Examining the curves of the three 
deeper thermometers, it will be seen that they reach the 
lowest temperatures at different periods, one at six feet 
about three weeks behind the one at three feet. It is about 
six weeks behind the surface temperature, though in this 
case the accidental and short period variations mask the an- 
nual curve at the surface. During the summer there is a 
corresponding lagging of the highest temperatures at the 
greatest depths, though the normal curve is altered by the 
irrigations to which the ground where the thermometers 
were placed was subjected. 
