If a fluid is flowing horizontally and along a section of a streamline, where the speed

increases it can only be because the fluid on that section has moved from a region of higher pressure to a region of lower pressure: and if its speed decreases, it can only be because it has moved from a region of lower pressure to a region of higher pressure. Consequently, within a fluid flowing horizontally, the highest speed occurs where the pressure is lowest, and the lowest speed occurs where the pressure is highest. Hence, in a steady horizontal flow, the pressure is greatest where the speed is least.