I do agree that it is a hydrometer. You know, I considered what you said also hogan...my brother makes his own beer and has a thermometer that could resemble this of sorts....But with the "glue liquor" mentioned on the instructions...they also had to measure the temperature of glues and solids of glues....I opted to go with my initial finding.
I still believe that is what this particular one was used for. Look at
http://www.madehow.com/Volume-5/Glue.html under the section "The Manufacturing Process - Making Hide or Skin Glue - #2" and it refers to glue liquor. In fact, if you search for glue liquor it will bring this up repeatedly about cooking/temperature, etc. I do believe that this fellow found a way to specialize this one for a particular industry.
I found an old article...to throw a twist on this....that could have been what this was used for as well... search title "Resources of the Southern Fields and Forests, Medical, Economical ..." and quoting "With linseed oil and common glue, a waterproof material is made, which may prove of great use in preparing garments for our soldiers. Immerse common glue in cold water until it becomes perfectly soft, but yet retaining its original form; after which it is to be dissolved in common raw linseed oil, assisted by a gentle heat, until it becomes entirely taken up by the latter; after which it may be applied to substances for adhesion to each other, in the way common glue is usually applied. It dries almost immediately, and water will exert no action upon it. It has more tenacity than common glue, and becomes impervious to water. It may be used also for furniture, and two layers of cloth may be glued together to form a water-proof garment. Glue dissolved in vinegar also makes a very tenacious substance in place of the prepared glues. See plates of machinery for pressing linseed and other oils, Ure's Dictionary of Arts, article “Oils;” also Wilson's Rural Cyc., articles “Flax” and “Linseed.” The processes are described with plates. Those interested may find there a full statement of the method of gathering, planting, uses, etc. See also “Olea,” in this work. Flax seed intended for planting should not be gathered too quickly. Flax seed was largely made in western New York. The yield is from ten to fifteen bushels per acre. It is sown early in the spring. If raised merely for the seed, it is harvested and thrashed like other grain. But when the stalk is used, it is pulled" " I see a notice of its being used by Mr. Cummings for this purpose...."
SO....maybe Cummings who lived in NY was the one this referred to?!!?!?? No more mention and no more information!