WASHINGTON -- Generously listed at 5-6, 165 pounds in the media guide, Daniel Herrera, the Mets' newly acquired relief pitcher, can't produce a great deal of velocity on the mound.

"I don't have a lot of heat," Herrera said. "I wish I did."

What he does possess is something more unusual: The lefthander throws a screwball, a pitch famously associated with the likes of Christy Mathewson, Carl Hubbell and, in the last half-century, Tug McGraw and Fernando Valenzuela.

The Nationals got a good look at Herrera's not-so-secret weapon the past two nights and couldn't do much with it, although the Mets managed to stumble to an 8-7 loss last night. Bobby Parnell blew the save in the ninth inning on Ryan Zimmerman's walk-off two-run single, a broken-bat blooper to right with the bases loaded and one out. The Mets intentionally walked Roger Bernadina to get to Zimmerman and Terry Collins said they had no choice but to try for the double play.

"I didn't want to," the manager said. "But Bernadina was going to put the ball in play and we would wind up facing Zimmerman, anyway."

Earlier, Jason Bay snapped a franchise record 0-for-35 road slump with a tying two-run homer and Nick Evans went deep for the second time in as many games. Angel Pagan had two RBIs and Lucas Duda snapped a 6-6 tie with a sacrifice fly in the seventh inning. The Mets, who entered the game having won three straight and seven of eight, have hit five homers in the first two games of this series after not going deep once during their six-game homestand.

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Despite Herrera's reluctance to use the screwball against lefties -- the reverse curve breaks toward them -- Nats manager Davey Johnson pinch hit for Rick Ankiel twice.

On Friday, Herrera struck out Jonny Gomes on a 67-mph screwball to end the Mets' 7-3 win. Last night, with the score tied at 6, one out and runners at first and third in the sixth inning, Herrera got Ivan Rodriguez to ground into a double play -- again with a 67-mph screwball.

Herrera and righthander Adrian Rosario were sent to the Mets on Thursday to complete the Francisco Rodriguez trade.

"I get a lot of swings off it just because I've been around a little bit, guys know me and they just sit on the off-speed stuff," Herrera said. "Now that's what all the hitters look for."

Herrera, 26, didn't find it until 2005, when he spent a college summer pitching in the Northwoods League in La Crosse, Wis. Looking to dump his changeup, Herrera spent months experimenting with the screwball, which requires a harsh torque of the elbow and wrist.

"I kept messing with different grips," said Herrera, who also throws a slurve, cut fastball and sinker. "I kept pronating the ball over to where the spin didn't spin like a changeup. It spun over the top like a curveball. It was just kind of a trial-and-error deal. It took a while to hone in and throw it for strikes and actually be effective."

When pitching coach Dan Warthen was asked about the pitch, he mimicked the twisting delivery with his arm, grimaced and grunted. To most people, it hurts just to try.

"Knock on wood -- my body's held up, my arm's held up," Herrera said. "That pitch has got me to where I am. It gave me the jump-start I needed. It's kind of been a savior for me."