TORRICELLI

TORRICELLI

In the closing years of his life Galileo took into his family, as

his adopted disciple in science, a young man, Evangelista

Torricelli (1608-1647), who proved himself, during his short

lifetime, to be a worthy follower of his great master. Not only

worthy on account of his great scientific discoveries, but

grateful as well, for when he had made the great discovery that

the "suction" made by a vacuum was really nothing but air

pressure, and not suction at all, he regretted that so important

a step in science might not have been made by his great teacher,

Galileo, instead of by himself. "This generosity of Torricelli,"

says Playfair, "was, perhaps, rarer than his genius: there are 13113t1911n

more who might have discovered the suspension of mercury in the

barometer than who would have been willing to part with the honor

of the discovery to a master or a friend."

Torricelli's discovery was made in 1643, less than two years

after the death of his master. Galileo had observed that water

will not rise in an exhausted tube, such as a pump, to a height

greater than thirty-three feet, but he was never able to offer a

satisfactory explanation of the principle. Torricelli was able to

demonstrate that the height at which the water stood depended

upon nothing but its weight as compared with the weight of air.

If this be true, it is evident that any fluid will be supported

at a definite height, according to its relative weight as

compared with air. Thus mercury, which is about thirteen times

more dense than water, should only rise to one-thirteenth the

height of a column of water--that is, about thirty inches.

Reasoning in this way, Torricelli proceeded to prove that his

theory was correct. Filling a long tube, closed at one end, with

mercury, he inverted the tube with its open orifice in a vessel

of mercury. The column of mercury fell at once, but at a height

of about thirty inches it stopped and remained stationary, the

pressure of the air on the mercury in the vessel maintaining it

at that height. This discovery was a shattering blow to the old

theory that had dominated that field of physics for so many

centuries. It was completely revolutionary to prove that, instead

of a mysterious something within the tube being responsible for

the suspension of liquids at certain heights, it was simply the

ordinary atmospheric pressure mysterious enough, it is

true--pushing upon them from without. The pressure exerted by the

atmosphere was but little understood at that time, but

Torricelli's discovery aided materially in solving the mystery.

The whole class of similar phenomena of air pressure, which had

been held in the trammel of long-established but false doctrines,

was now reduced to one simple law, and the door to a solution of

a host of unsolved problems thrown open.

It had long been suspected and believed that the density of the

atmosphere varies at certain times. That the air is sometimes

"heavy" and at other times "light" is apparent to the senses

without scientific apparatus for demonstration. It is evident,

then, that Torricelli's column of mercury should rise and fall

just in proportion to the lightness or heaviness of the air. A

short series of observations proved that it did so, and with

those observations went naturally the observations as to changes

in the weather. It was only necessary, therefore, to scratch a

scale on the glass tube, indicating relative atmospheric

pressures, and the Torricellian barometer was complete.

Such a revolutionary theory and such an important discovery were,

of course, not to be accepted without controversy, but the feeble

arguments of the opponents showed how untenable the old theory

had become. In 1648 Pascal suggested that if the theory of the

pressure of air upon the mercury was correct, it could be

demonstrated by ascending a mountain with the mercury tube. As

the air was known to get progressively lighter from base to

summit, the height of the column should be progressively lessened

as the ascent was made, and increase again on the descent into

the denser air. The experiment was made on the mountain called

the Puy-de-Dome, in Auvergne, and the column of mercury fell and

rose progressively through a space of about three inches as the

ascent and descent were made.

This experiment practically sealed the verdict on the new theory,

but it also suggested something more. If the mercury descended to

a certain mark on the scale on a mountain-top whose height was

known, why was not this a means of measuring the heights of all

other elevations? And so the beginning was made which, with

certain modifications and corrections in details, is now the

basis of barometrical measurements of heights.

In hydraulics, also, Torricelli seems to have taken one of the

first steps. He did this by showing that the water which issues

from a hole in the side or bottom of a vessel does so at the same

velocity as that which a body would acquire by falling from the

level of the surface of the water to that of the orifice. This

discovery was of the greatest importance to a correct

understanding of the science of the motions of fluids. He also

discovered the valuable mechanical principle that if any number

of bodies be connected so that by their motion there is neither

ascent nor descent of their centre of gravity, these bodies are

in equilibrium.

Besides making these discoveries, he greatly improved the

microscope and the telescope, and invented a simple microscope

made of a globule of glass. In 1644 he published a tract on the

properties of the cycloid in which he suggested a solution of the

problem of its quadrature. As soon as this pamphlet appeared its

author was accused by Gilles Roberval (1602-1675) of having

appropriated a solution already offered by him. This led to a

long debate, during which Torricelli was seized with a fever,

from the effects of which he died, in Florence, October 25, 1647.

There is reason to believe, however, that while Roberval's

discovery was made before Torricelli's, the latter reached his

conclusions independently.