We propose a hydraulic user interface consisting of an array of spray jets and the appropriate fluid sensing and fluid flow control systems for each jet, so that the device functions as a fluid-based tactile user interface. Our array of fluid streams work like the keys on a keyboard, but where each fluid stream can also provide tactile feedback by dynamically modulating the pressure of the fluid spray, so that the keyboard is actually bi-directional (i.e. is both an input and an output device).

A 104-jet version can be used as a fun and tactile "QWERTY..." style keyboard. More importantly, however, the device can also be used for applications, such as musical instruments, where its more expressive multi-dimensional input capabilities can be put to full use.

One such instrument, the hydraulophone (hydrauliphone, hydraulaphone), is a hollow tubular object with a row of holes in it. It is played much like one would play a tin flute or recorder, by covering up the holes to restrict fluid flow.

This gives rise to a fun new way of playing music by successively blocking water jets in a fountain, or while frolicking in a pool, or splash pad. Additionally, the hydraulophone can be used as a teaching tool to help children learn music by playing in the water. We demonstrate this teaching capability by way of an implementation of the arcade game "Touch Me" using a hydraulic user interface.

We describe some of our present and upcoming installations of these devices in public parks, pools, and aquatic play areas.