Follis was the Latin word for bellows. Many Roman bellows were very much like the more 'modern' bellows you might be picturing; leather bags, sandwiched between two plates for easy pumping. However, they appeared in a range of forms and sizes, as the Romans were more than advanced enough to recognize the value of pressurized air, and various designs of bellows were used in smithing, pottery, and musical organs.
The word follis remains in English in two familiar words: follicle comes from folliculus, a 'little (bellows) bag'; and 'fool' comes more directly from follis, refering to a person whose head is filled with air, like a bellows.
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