U-tube came before YouTube

Seen in the hands of Joseph Black in the portrait gallery in Edinburgh ... This piece of equipment is why the name of the video channel on the Internet is a clever physics joke for those in the know. A u-tube usually contains a fluid like water. It can be used as a manometer to measure gas pressure by connecting one end of the u-tube to the gas and leaving the other end open to the atmosphere. If the gas has a pressure higher than that of the atmosphere, the liquid moves round to the atmosphere side. Vice versa if the situation is reversed. Or a u-tube of water with nothing attached can be shown to exhibit simple harmonic motion if the water is displaced to one side and allowed to oscillate.