TITLE: Where does the symbol for a partial deriviate come from?
QUESTION [5 upvotes]: Does anybody know where the symbol $\partial$  comes from?
(preferably with sources or with a document where it was used first)
Symbol in context:
$$f\colon \mathbb{R}^2\rightarrow \mathbb{R}$$
$$f(x,y):= x^2 + y^2$$
$$\frac{\partial f(x,y)}{\partial y} = f_y = \lim_{h \to 0}\frac{f(x,y + h) - f(x,y)}{h} = 2 \cdot y$$

REPLY [2 votes]: See this, where you find explicit references to the publications where this symbol appeared.