If Rube Goldberg ever designed a slide guitar, it would look like this.

This monstrosity is the Hawaiian Tremoloa, a strange fretless zither first patented in 1932 and produced by the Oscar Schmidt factory until the 1950's.  

Tremoloa patent reprint posters: Just $7.50

The Tremoloa was created to cash in on the Hawaiian music craze that swept the US and features a very peculiar slide contraption, supposedly to enable every novice to become the next Sol Hoopii.  

Unfortunately, these things are almost impossible to play!

Here's two people wrestling a simple melody on a Tremoloa:

There's a single melody string which is under fairly low tension. A sliding (or rolling) weight rests on the string and is positioned by the player to locations marked on the large note label under the string. That position controls the pitch of the string when plucked.

The relatively heavy weight actually bounces on the string and alternately raises and lowers its tension a little. Thus the pitch is raised and lowered a little around the desired pitch, and the sound becomes a broad vibrato. In addition, as the weight moves from one played note to the next, the weight stays in contact with the string, and creates a glissando that is reminiscent of the sound of a Hawaiian steel guitar.  (Source: Fretlesszithers.net)

In addition to playing the slide with their right thumbs, Tremoloa players had to strum chords with their left hand:

The majority of these instruments were sold door-to-door by dubious salesmen in rural America.  Thousands and thousands were sold.  Very few people ever mastered them.  You can find Tremoloas on eBay for under $100.