Many thanks to both of you, David and Pete!!
Obviously, I forgot how to use the water gauge properly! Mea culpa! You are both utterly correct. I was expecting to find 5 inches of pressure and could not understand why I was reading 2-1/2 inches instead. LOL!
Moving along to my next issue: When I tried to read the static pressure of the organ, which is somewhere between 10 and 15 inches, the water just blew out of the top of my gauge and gave me a mini shower. Perhaps I should carefully put in just enough water for the "U" to have room to go down 8 inches on one side and up 8 on the opposite side? Perhaps too little water just blows out and too much water would also overflow, because the gauge needs room to show a differential of up to 15 inches?