Why did the military switch it's story so quickly regarding the Roswell crash?

The military issued a press release on July 8th, 1947 indicating that they had recovered the wreckage of a flying saucer; the following day they retracted their original statement and said it was a weather balloon. Wouldn't the military have all the facts straight before issuing such a bold press release? Wouldn't they have been able to discern a weather balloon from something that is purportedly as advanced as a flying saucer?