66 Dr. Lathrop’s account of the spring's and wells in Boston. 
peninsula may be found nearer home. The ponds at the northward, 
at the west, and southward, have a sufficient elevation, and, as reser 
voirs, contain quantities of water, sufficient to furnish innumerable 
springs, between them and the sea. Let us suppose, that under 
some pond, several miles from Boston, there is placed a stratum of 
clay, which serves as a bason to prevent the water from sinking into 
the earth, and that next to the stratum of clay there is placed a vein of 
gravel, and over that clay again, or hard earth, (as we find strata com- 
monly disposed) and we may conceive of a complete aqueduct, from 
the pond to the sea. If the pond be very deep, veins of sand or grav- 
el, between strata of clay, at different distances from the surface, 
may furnish supplies for the springs on the elevated parts of the pen- 
insula, as well as. for those whick are. found at. 100 or 120 feet under a 
bed of clay. 
In the drawing annexed, the pond and stratum of oat between 
strata of clay, may be considered as one leg of an inverted syphon ; 
the well dug in the side of the hill, and which just enters the vein of 
gravel and water, may be considered as the other leg. The pressure 
on the pond would raise the water in the well to the same level, if the 
syphon was complete. But it is to be remembered, while a part of 
the water is forced up the well, where the passage is easy, probably 
much the greatest part, which comes from the source, is still carried 
along in the vein of gravel. It is impossible for us to say why the wa- 
ter holds a certain elevation, and rises no higher, while we are unac- 
guainted with the degree of obstruction, which it meets with in its ori- 
ginal course. Ifthe obstruction is great, so that. the water has very 
little motion, where the vein is opened, it will rise high in the well ; 
but if the passage. is comparatively easy, the depth of water in the well 
will not be great. At a certain’elevation the water in the well will be 
abalance for the pressure at the source, (allowing always for the force 
