Dow Jones to Buy MarketWatch  LONDON (Reuters) - Dow Jones   Co. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=DJ.N target=/stocks/quickinfo/fullquote"&gt;DJ.N&lt;/A&gt;, publisher of the  Wall Street Journal, said it is to buy online news site  operator MarketWatch Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=MKTW.OQ target=/stocks/quickinfo/fullquote"&gt;MKTW.OQ&lt;/A&gt; to grow the revenues it  derives from the Internet.