Found on the Science Museum UK website.

“This portable spirometer is made of brass, rubber and glass in a mahogany case. It was made by Arnold and Sons of London. The patient’s breath bubbled up a water-filled collecting glass and pushed up a sliding weight. A scale on the attached spirometer indicated the capacity of the lungs. Scientific instrument maker, Robert Mann Lowne (1844-1929) patented this portable spirometer in 1865.”