|Let||x denote all steamers, or steam-vessels,|
|y denote all armed vessels,|
|z denote all vessels of the Mediterranean.|
(1) That there are no armed vessels except steamers in the Mediterranean.
(2) That all unarmed steamers are in the Mediterranean (since the steam-vessels not of the Mediterranean are armed). Hence we infer that the vessels of the Mediterranean consist of all unarmed steamers; any number of armed steamers; and any number of unarmed vessels without steam. This, expressed symbolically, is equation (15).