Northamptonshire 180 (Vasconcelos 78, Overton 4-50, Henry 3-36) and 151 for 6 (Procter 37*, Henry 3-36, Overton 2-39) f/o trail Somerset 351 (Dickson 70, Bartlett 55, White 5-77) by 20 runs

Somerset's seamers were in ruthless mood as they bulldozed Northamptonshire's fragile batting line-up and enforced the follow-on to move within sight of victory at Wantage Road.

The home side stumbled to 151 for 6 second time around before bad light brought play to a halt, still needing 20 more to prevent what would be their fifth innings defeat this season.

Henry was a handful straight away when Northamptonshire began their response to the visitors' first-innings total of 351, making the ball move both ways and persuading Emilio Gay to nibble the fifth delivery of the morning into James Rew's gloves.

Despite registering successive boundaries off Henry, Justin Broad soon followed - forced to play at one that left him and edging to first slip to leave the home side in difficulty at 17 for 2.

There was also a near escape for Vasconcelos, his thick edge off Overton whistling through the slips for Northamptonshire's first scoring shot in 31 balls but, despite plenty of playing and missing at Henry, he weathered the storm.

The left-hander looked far more comfortable against Somerset's change bowlers, drilling a series of crisp boundaries through the off side - and the 10th of those, sweetly struck off Overton, brought up his half-century from 76 balls.

By then he had lost Sam Whiteman, playing on to Lewis Gregory and Henry returned for a pre-lunch spell to keep Somerset in the ascendancy by prising out Luke Procter, taken low at first slip.

Vasconcelos continued in similar vein after the interval, looking on course for a first century of the season until he was undone by a ball from Gregory that veered away to hit off stump - triggering a further surge of wickets.

Overton was the chief architect of Northamptonshire's collapse during a fearsome second spell, having Rob Keogh caught in the slips and ripping out the off stump of Lewis McManus before Ben Sanderson feathered one behind.

Running out of partners, Saif Zaib did his best to propel the home side past the follow-on target with a lively knock of 38, including two colossal sixes off Kasey Aldridge, but fell short when he skied Overton and was taken by Shoaib Bashir, running back on the leg side.

Somerset had no hesitation in putting the hosts back in and Vasconcelos was soon on his way for the second time in the session, caught behind off Henry - who also trapped Gay lbw shuffling across.

With Broad also departing leg before to Gregory, Northamptonshire sank to 29 for 3, but Procter and Zaib - back at the crease barely half an hour after his dismissal - displayed some grit to slow the Somerset charge.

However, neither Zaib nor Rob Keogh, another leg before victim for Henry, managed to build on promising starts and it was left to the skipper to tough it out with an undefeated 37 while Taylor went for his shots.

The all-rounder clubbed Overton for successive boundaries to reach a brisk 24 not out from 17 balls before darkening skies prompted the umpires to halt proceedings with 8.4 overs still remaining.