closed instrument was then graduated by degrees marked on the stem.
This was the first thermometer independent of atmospheric pressure. Torricelli, it will be remembered, had shortly before invented the barometer and demonstrated the weight of the atmosphere. Tuscan savants and Blaise Pascal had applied it to the measurement of elevations. In constructing this new thermometer Ferdinand probably was guided by an experiment made by certain Florentine savants to show the influence of atmospheric pressure. The latter took a U-shaped glass tube open at both ends, on one arm of which two bulbs were blown, the uppermost ending in a very small open point; at the foot of a high tower the U-tube was filled to a certain point with liquid that reached the same height in both arms, and the open point was then closed by melting the glass, care being taken not to warm the air in the bulbs. The whole apparatus was then taken to the top of the tower and the liquid was seen to rise in the open arm and to sink in the closed arm owing to diminished pressure.
Ferdinand also applied the principle of the Cartesian divers to the construction of a thermometer, devising an entertaining apparatus