NEW YORK, Sept 24 (Reuters) - Short interest on the New York Stock Exchange rose 3.3 percent in mid-September, the exchange said on Wednesday.

As of Sept. 15, short interest rose to about 19.181 billion shares, compared with 18.568 billion shares as of Aug. 29.

Investors who sell securities short borrow shares and then sell them, expecting the stock to fall so they can buy the shares back at the lower price, return them to the lender and pocket the difference.