The Fitzroy Stormglass was invented by Admiral Robert Fitzroy. The glass vessel contains a mixture of distilled water, ethanol, potassium nitrate, ammonium chloride, and camphor.
It is not explained how or why the stormglass actually works, as the physical changes cannot be explained from the conditions proceeding the weather event that is forecast. The basic explanation is that the pressure and temperature cause the liquid to change it's solubility, thus causing patterns of crystals to form within the tube.
I purchased my Stormglass from Weather in a Bottle.
The formation of these ferns should forecast windy conditions, in approx 2 days. This was quite accurate as we did experience windy conditions around 2 days later.