professor .

Some economists do believe that the government interventions into the economy under Hoover and Roosevelt prolonged the Depression.  However, since there were also two severe depressions in the late 1870s at a time where government did very, very little to try and regulate the economy, I think the idea that depressions are caused by government interventions is a bit shaky.

Consider as well that while the U.S. economy suffered three major depressions from the 1870s to the 1930s (approximately one every 20-30 years), we haven't had a depression since.  Is this because of the mechanisms put in place by the New Deal? Perhaps..

The interventions under the New Deal were less than successful in ending the Depression, and as you note, probably prolonged it.  However, these measures were designed to correct what were seen as two key longterm problems in the economy:

1/ Financial instability created by the collapse of the stock market and banking system (which had absolutely nothing to do with government intervention).  Simply put, without liquid capital, capitalism doesn't work.  The New Deal attempted to fix this problem by instituting government controls to regulate the behavior of the stock market (SEC) and banking (Glass-Steagall -- FDIC)to prevent overspeculative investing (the subprime bubble is an example of this type of investing).

2/ Imbalance between consumption and production.  By the 1920s American industry had reached a pinnacle of production capability, but real wages lagged behind.  Henry Ford himself had very early on recognized this as a fundamental problem in the American economy -- which is in part why he paid his workers high wages (so they could afford the product he was producing).  Again, a capitalist economy doesn't work very well if you don't have a market.  Despite some gains in real wages, the fact is that much of the prosperity of the 1920s was based upon consumer debt (just like today -- much of our prosperity seems based upon consumer, corporate, and government borrowing).  Many of the measures of the New Deal sought to correct this imbalance by putting more money into the pockets of workers -- through minimum wage laws, support for unions, and Social Security.  

You might define this as artificial tinkering with the market -- but keep in mind that wearing clothes is also not "natural".  Man manipulates his environment.  The free market is a powerful idea, but I don't think it has ever existed in a pure form.  Look at the late 19th century where the great industrial barons of the age happily manipulated the government to get subsidies, high tariffs, etc. to support their industries.