From The Collaborative International Dictionary of English v.0.48:
Doubler \Dou"bler\, n.
1. One who, or that which, doubles.
[1913 Webster]
2. (Elec.) An instrument for augmenting a very small quantity
of electricity, so as to render it manifest by sparks or
the electroscope.
[1913 Webster]
3. A part of a distilling apparatus for intercepting the
heavier fractions and returning them to be redistilled.
[Webster 1913 Suppl.]
4. (Calico Printing) A blanket or felt placed between the
fabric and the printing table or cylinder.
[Webster 1913 Suppl.]