Glass specific gravity hydrometer by J W Towers
1920-1960
1920-1960
1930-1960
1930-1960
1775-1800
1934-10-23
1934-10-23
1934-10-23
1935
1921-1930
1957 (original); 1961 (model)
1966
1911
1943
1960-1970
1920-1960
1941
1920- 1960
1920-1960
1920-1960
1934-03-22
1964-08-31
1920-1960
1920-1960
1996
1920-1950
1993-1998
1966
1966
1966
1966
1910-1920
before 1889
1968
1994
1969
1920-1960
1900-1980
1984
1970
20th century
1920-1960
1982