Nat Sciver was unbeaten on 169 when England declared, before Issy Wong gave the hosts the advantage at the close
Taunton (third day of four): South Africa 284 & 55-3, England 417-8 - South Africa trail by 78 runs with seven second-innings wickets remaining
Poor weather on the third day at Taunton initially prevented England from taking full advantage of their dominance against South Africa until Issy Wong picked up two late wickets to leave the hosts in charge.
The debutante fast bowler had Lara Goodall caught down the legside and Laura Wolvaardt at gully in a late burst after both teams had been forced to wait until 6.30pm for an evening restart following afternoon rain.
Earlier, Nat Sciver had added fifty runs to her overnight 119 and was unbeaten on 169 when Heather Knight declared half an hour before the lunch interval.
But only 4.1 overs were possible until rain curtailed the session, in which Andrie Steyn fell to Kate Cross for the second time in the match. The teams returned to the field once the clouds had cleared, but after a further 7.5 overs, the rain came back.
Come the evening, when play resumed following multiple inspections, 12.5 overs were scheduled and initially passed without incident.
But Wong worked up ahead of steam once into her spell and benefited from a pitch that gained pace from a period of steady drizzle. Once Goodall fell for 26, Wolvaardt was well caught by Sciver before Sune Luus and nightwatcher Tumi Sekhukhune survived the final throes of an electric mini-session; Wong conceded just eight runs in her six overs and justified her hype as a potential difference-maker.

The weather at Taunton wreaked havoc on Wednesday (Harry Trump/Getty Images)
For her part, Sciver was imperious, punching her first ball of the morning through mid-on and ticking over the scoreboard at will. Sophie Ecclestone, who joined her after Alice Davidson-Richards' departure to the last delivery of the second evening, began tentatively but added four boundaries of her own en route to her highest international score.
She fell when Anna Harris – who has enjoyed a fine game as umpire – yet again ruled correctly on an lbw appeal as Nonkululeko Mlaba skidded an arm-ball into Ecclestone's pads.
In the same over, Cross was run out by a Nadine de Klerk direct hit at the non-striker's end, leading to a declaration with Sciver 31 runs short of England's first Test double hundred.