This is a ligature of two 's' characters, one following another.
It was in common usage in English as well, until the end of the 1700s.
A single "s" character was often drawn as an "f" without the crossbar. When two consecutive "s" characters were drawn, they were combined into the form about which you are asking.
In English, the first character morphed into the "s" glyph during the Napoleonic Wars. The ligature then disappears from contemporary English usage, but persisted in German.
In Unicode, while ligatures are provided as convenience characters, the standard strongly recommends that ligatures are decomposed into their component character points, while encoding them as a ligature. Unicode is about encoding things as characters, not as glyphs. The Unicode standard shows the ligature of "f" and "i" as its canonical example of ligatures, in Section 2.2.
Examples of the morph of the long s and the fs ligature in English can be found in "The Art of Defence on Foot", by Taylor and Roworth. There are three revisions, 1798, 1804 and 1824. The revisions between the 2nd and 3rd editions are almost entirely changes to the typesetting of the long 's'. (The 1804 Edition also makes extensive use of the 'ft', 'fh' and 'ct' ligatures, and even the 'fk' and 'ff' ligatures).
[ed. Steven B. Segletes] The fs ligature is shown in the following, well known document (circa 1776), both in the words "necessary" and "Happiness" which appear about mid column in lines 1 and 4. Note: it is also used for words beginning in "s", such as "secure" on line 4.
And as far as printed copies, it is likewise so (though presented as both an "ff" in "diffolve"and an "fs" in "Happinefs"), as seen in the first printings of the Declaration (ref: http://www.ushistory.org/us/10g.asp):