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POPULAR SCIENCE REVIEW. 
2,000 fathoms, that their thermometers indicated a tempera- 
ture of 39 or 40 degrees, we now know that the smallest error of 
their thermometers being seven or eight degrees at those depths, 
the true temperature could not have been higher than about 
32 degrees — that is, about the freezing point of fresh water. 
The means which Professor Miller suggested for overcoming this 
difficulty was extremely simple. It was merely to enclose the 
bulb of the thermometer in an outer bulb, sealed round the 
neck, a space being left between the two bulbs. Now that 
space was not left entirely empty ; it was about three parts 
filled with fluid. You may ask, Why was the fluid introduced 
there? For this reason — if only air had been left in that 
space, the inner bulb would have been a very long time in 
taking the temperature of the water round the outer bulb ; the 
air being a bad conductor, it would have been necessary to 
allow the thermometer to remain perhaps an hour before the 
mercury or spirit of the inner bulb would have taken the 
temperature of the water outside ; but by introducing between 
the bulbs some spirit, that spirit conveyed the heat or the cold 
from the outer to the inner. Still the intervening space was 
not filled with the spirit, because if it had been, the pressure upon 
the outer bulb, and its consequent change of form, would have 
acted in the same manner upon the inner bulb ; but by leaving 
void a part of that space, any reduction in the capacity of the 
outer bulb which pressure might produce merely diminished 
that void, and produced no alteration in the shape of the inner 
bulb. We subjected thermometers, which were thus protected, 
to the pressure of three tons to the square inch, and found that 
they did not rise more than about one degree ; and that small 
rise was really due, we have reason to believe, to an increase 
of heat in the liquid occasioned by the pressure to which it was 
subjected. That is the mode in which the thermometer has 
been adapted to the purpose of obtaining the real temperature 
of the deepest ocean waters ; and I shall show you what very 
important information we have derived from its use. 
The pressure which is caused by a column of water of course 
varies with the height of the column — that is to say, with the 
depth of the water ; and in round numbers we may say that at 
800 fathoms the pressure of a column of water is one ton upon 
every square inch ; therefore, at 2,400 fathoms, which was 
nearly the greatest depth to which our soundings extended, the 
pressure is three tons to the square inch ; and that is just the 
pressure to which our thermometers had been tested. There- 
fore we know that we had within a degree (we always used two 
thermometers) the real temperature of the bottom of the ocean. 
Now I shall show you what very curious and important informa- 
tion we derived from ascertaining the temperature, not only of 
