A thermostat disposed to operate in a direction that is normal to the direction of flow of the fluid for which the thermostat regulates the temperature. In one embodiment, the thermostat comprises two mating faces, their movement being controlled either by the expansion or contraction of the carefully selected and apportioned materials in the thermostat pack or by a sensor-triggered controller in response to the temperature of the fluid to which the thermostat is exposed. The first type or Type 1 is normally closed, typically for use in liquid-cooled engines. The second type or Type 2 is normally open, typically for use in industrial applications, such as in solvent evaporation/recycle units where it is desirable to stop direct coolant flow into a paint gun cleaner or a similar equipment, when the temperature of the coolant reaches beyond a pre-determined upper limit.