Rotating glasses into a trough of water
The idea is to have the armonica glasses rotate through a trough of water so the player doesn't have to wet the glasses manually. Various sources state that the trough of water was indeed a feature of historical glass armonica. Some assert that this 'feature' was also present on Franklin's armonica, but Franklin did not prescribe one, nor is one present on Franklin's own armonica. As any armonica player can atest, 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. Furthemore, the left hand can re-moisten the lower glasses while the right hand keeps playing, and vice versa.
Singers figure out where to breathe, armonia players figure out where to dip.
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.
The problem is, as I've verified by trying it for myself, rotating a 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. All of this to solve a 'problem' which was never a problem in the first place.
Please see this video for a demonstration of the effect of rotating a musical glass through a trough of water.
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:
a) Only someone who has never tried to tune a set of armonica glasses could think that this is possible within the realm of sanity.
b) The water level must be extraordinarily precise. Evaporation is unacceptable.
c) Now we're right back to the water tuning issue that Franklin was trying to avoid.
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!