How can one draw a conclusion about the existence of atmospheric pressure from the results of Torricelli’s experiments?

Mercury was not poured out of the open end of the tube because the force of gravity acting on the mercury was balanced by the force of atmospheric pressure.

The Italian scientist E. Torricelli first guessed how to measure the pressure of the atmosphere. The experiment he proposed was carried out in 1643 by Galileo’s student V. Viviani. In this experiment, a glass tube about 1 m long, sealed at one end, was used. It was filled with mercury and, closing it with a finger (so that the mercury did not spill out ahead of time), it was turned over and dipped into a wide bowl with mercury. After the tube was opened, part of the mercury poured out of it, and an airless space – “Torricellian void” was formed in its upper part. In this case, the height of the column of mercury in the tube turned out to be about 760 mm (if we count it from the level of mercury in the bowl).

Torricelli explained the results of this experiment as follows. “Until now,” he wrote, “there was an opinion that the force that prevents mercury, despite its natural property, from falling down, is inside the upper part of the tube, that is, it consists either in emptiness or in an extremely rarefied substance. I say that this is a force – external – and that the force is taken from the outside. The surface of the liquid in the bowl is influenced by its weight 50 miles of air. What is surprising if the mercury … rises enough to balance the weight of the outside air. “