A trough was sometimes placed beneath the row of glasses. Many sources assert that this was filled with water so the glasses would remain constantly wet.1
As any armonica player can atest, however, keeping the glasses moist is simply not a problem. One good 'dip' at the beginning is sufficient for most pieces. For longer works like the Mozart Rondo, there are more than enough places where the armonica rests while the rest of ensemble keeps playing—in these 'rest' intervals the armonica has more than enough opportunity to remoisten the glasses—if needed. And the player also has the option of re-moistening the lower glasses with the left hand while the right hand keeps playing, and vice versa.
Singers figure out where to breathe, armonica players figure out where to dip. One of the easier aspects of playing the instrument. Nevertheless, non-armonica-players frequently assume that keeping the glasses moist must be a 'problem' that they must solve for us. One frequently proposed solution is to have the glasses rotate through a trough of water.
So I tried it! I mounted an armonica glass we made on a glass lathe, set the speed appropriately, and then lifted a basin of water to immerse the rim to various depths.
And I discovered: rotating the glass into a trough of water has exactly the same effect as adding water to a wineglass—it lowers its pitch. And since each glass on a glass armonica is a different size, each glass will rotate into the trough with a different depth, and will consequently have its pitch altered by a different amount. In other words, rotating several dozen armonica glasses (on a typical armonica) into a trough of water will result in musical chaos.
Furthermore, when the glass was rotating through the trough of water, it was harder to play—the water dampened the sides of the glass and made it noticeably harder to make it speak.
The water trough also muffled the sound of the glass.
Some might suggest the glass armonica maker have the glasses appropriately out of tune so that when the trough is filled, they are put back in tune. I can only say that:
- As someone who has tuned sets of armonica glasses—they are difficult enough to tune as it is without the water trough. The mind boggles at what would be necessary to pre-compensate for the effect of the water trough.
- The water level must be extraordinarily precise. Evaporation is now a significant problem.
- Now we're right back to the water tuning hassle that Franklin was trying to avoid.
So let's see now: in order to avoid having to dip once or twice in the course of a piece we're going to:
- Have the hassle of filling the trough to a very precise level
- Make the glasses more difficult to make speak
- Make the glasses sound muffled
- Make tuning very problematic if not outright impossible
All of this to solve a 'problem' which was never a problem in the first place. In other words, the water trough is just one of those 'good ideas' that simply wouldn't work in practice.
There are indeed armonicas with a 'trough'. Might I suggest that the real purpose of the empty trough was to catch the inevitable water drips? That would actually be useful!