210 
POPULAR SCIENCE REVIEW. 
the waters of the Green Sand into the sea. Not only is the termi- 
nation of a porous stratum in the sea common enough^ but it 
sometimes occurs under such circumstances as to give rise to the 
singular phenomenon of fresh^ or even mineral springs, rising in 
the sea, and preserving their identity for some distance from 
their place of origin. Such a spring is mentioned by Hum- 
boldt as occurring at the mouth of the Rio Lagartos, off Cape 
Caloche, 400 yards from land. Another is quoted by 
Spallanzani as existing in the sea off Spezzia, and troubling the 
waters to such an extent that a boat, when over it, is violently 
agitated. Between Castellamare and Sorento there is a’ 
sulphuretted spring in the sea ; and the inhabitants of Syracuse 
enjoy the paradox of supplying themselves with freshwater 
from a spring which rises in the sea near their town. 
All springs may be divided into cold and warm, or, as they 
are often called, thermal ; and this division bears an intimate 
relation to their geological origin, for the deeper the source of 
the spring, the higher is its temperatm’e. Taking the mean 
annual temperature of the air as the standard of comparison, 
it has been found that the majority of springs are thermal, i.e., 
their temperature is above the mean of the air of the locality 
above them. The cause of this elevation of temperature is 
easily explained. It is a fact well established by a large 
number of observations, that the temperature of the earth 
increases the further we descend below its surface. The only 
exception to this law occurs in tropical countries, and in tem- 
perate ones during the heat of summer. In both of these 
cases the superficial portion of the earth^s crust is heated by 
the solar rays ; but as their influence gradually diminishes in 
proportion to the depth we descend into the ground, we soon 
reach a point where it ceases altogether. This point varies in 
depth in different parts of the globe, according to the degree 
of latitude, and its extension over the globe forms what is 
called the surface of constant teni'perature. The exceptional 
decrease of temperature which takes place from this cause is 
shown in a well at Bahia, 200 feet deep, the temperature of the 
water in which is only 68° F., whilst that of the surface of the 
ground is 81° F. But although it is an undoubted fact that 
there is a gradual rise of temperature as we descend into the 
earth, u the rate of increase is by no means uniform. It is, 
indeed, stated in most text-books that it occurs in the propor- 
tion of 1° F. for every fifty feet of descent ; but even if this 
could be reckoned on as the average, the limits of fluctuation 
are very wide. It was found that in nine Prussian mines the 
increase varied between 1° F. in 21*3 feet, and in 155’5 feet. 
In some mines in Saxony, in the granite, the average increase 
was 1° in 5 7’ 3 feet ; whilst in one in the tertiary formation in 
