How to create a tnt cannon

A view of the finished product

Build two columns of obsidian, each one 7 block long. Make sure that there is a one block gap between then.

Connect the two columns by placing one piece of obsidian at on end

Put one lever on each side of the columns like in the photo

Place a block of obsidian on the end of the column on the right, as shown above

Place redstone dust on all the blocks of obsidian apart from the one between both levers

Place a slab of your choice on the ground, connecting the two columns of obsidian as shown in the photo above

Place a bucket of water in front of the block of obsidian without redstone dust on

Place four TNT blocks like so

Place a TNT block on top of the slab

You have now constructed your TNT Cannon! To activate the cannon, flick the lever on the left first, and then flick the lever on the right