122 
On Water and Air. 
[February, 
up to where that vessel stands. He has placed behind it an 
eleCtric lamp. It gives an intense beam of light. I want to 
show you, first of all, simply the vein of water fly nig out of 
this vessel. Mr. Cottrell will send into that vein a com 
densed beam of light. The light goes into that vein and 
cannot get out of it, and will be carried down with the 
water, and I hope that you will see the vein illuminated by 
the internal refleftion of the light. At the back of the vessel 
there is a little glass window, and through that window the 
beam is sent, so that it will fall exaftly upon the orifice at 
which the vein of water will run out. Mr. Cottrell will 
withdraw the cork from the vessel and the illuminated water 
will rush down. [The theatre was darkened, and upon the 
cork of the vessel being withdrawn the illuminated water 
descended like a stream of liquid fire. > Fig. 7.] 
Well, here you have a continuous vein flowing out in this 
wav, and forming the curve which mathematicians call a 
parabola. The great philosopher Galileo, making experi- 
ments at Pisa on the celebrated leaning tower of which all 
of you have heard, established what we call the laws of 
falling bodies. He determined the laws obeyed by bodies 
when they fall, and his celebrated pupil, Torricelli, connected 
those laws of Galileo with the outpour of a vein of water of 
this kind. You will understand immediately the law which 
rules this vein. If I allow a leaden ball to fall from a certain 
height to the ground it reaches the ground with a certain 
velocity. If I allow it to fall from a greater height it reaches 
the ground with a greater velocity. If I allow it to fall from 
the top of the house it would reach the ground with a still 
greater velocity ; and it is on this account that bodies falhng 
from a great height are more destructive than bodies which 
fall from a lesser height. _ Now, Torricelli proved th at the 
velocity of the water issuing from that orifice is exactly th 
which would be acquired by a body falling from the top of 
the vessel to the orifice. From that velocity it is able to 
describe this beautiful parabolic curve. 
Now we have to consider other veins which, unlike that 
we have just seen, are not continuous. If you allow water 
to flow from an aperture in the bottom of a tin vessel, you 
will find that the vein of water divides itself into two distinct 
portions. You have first of all a portion that appears 
feCtlv tranquil — almost as tranquil as a glass rod. A litt 
below that part you will find the rest of the vein unsteady , 
but the vein of water appears always continuous throughout, 
although one part is steady and the other unsteady. Now, 
