Our concept is that past the Earth’s atmosphere, in space, a vacuum exists. A vacuum can be defined as a space devoid of all matter and substance, a void per se. Another definition though is that a vacuum is a space in which the pressure is significantly lower compared to the pressure of our atmosphere. Blaise Pascal delved into this very subject and performed experiments which gave our desired proof and result. Pascal first grew interested by reading the results of the Torricelli experiments. In these Evangelista Torricelli wanted to experiment and explain the effects of a pump better, He created a pump that would inadvertently lead to his construction of the first barometer, and leading even further to Pascal’s experiments on our planet’s pressure.
Pascal repeated Torricelli’s experiments by creating his own with a glass syringe and a carefully fitted piston. While covering the open end with his finger he adjusted the height the syringe was submersed in the water, the empty space in the syringe changed accordingly proving to Pascal that nature abhors vacuums no matter what size it may be.
Later Pascal would perform another experiment to confirm the existence of air pressure. He asked his brother Florin Perier to carry the barometer up a volcanic peak. The mercury level 3000 feet above the base of the volcano was about three inches lower. They reached the conclusion that it was in fact pressure weighting down the mercury. From there he could state as one went high enough the air pressure would be lessened enough to allow a vacuum to be formed.
Here’s an article and how a Barometer works:
Palmer, Chad. "How a Barometer Measures Air Pressure." USATODAY.com. 5/20.2005. USA Today. 30 Oct 2008
And here’s a simple biography on Blaise Pascal himself and other things he invented.
Bellis, Mary. "Blaise Pascal (1623-1662)." About.com: Inventors. About.com. 30 Oct 2008 http://inventors.about.com/library/inventors/blpascal.htm