Leicestershire were reduced to 47 for two, still needing another 371 runs to avoid the follow on, before rain brought a premature end to the second day.
Northants added another 247 runs to their overnight 320 for four before declaring on 567 for seven in mid-afternoon. Kyle Coetzer made a career best 219, Andrew Hall (103) hit his third century of the season and James Middlebrook hammered a quickfire 84.
James Foster and Ryan ten Doeschate led a spirited Essex recovery as the home side reached 268 for eight in their match against Lancashire before bad light ended play early.
Both batsmen steered the home side towards a respectable total after they had lost half of their wickets for 72 before lunch at Chelmsford.
Ten Doeschate (77) initiated the recovery, reaching his half-century from 80 balls with six boundaries and adding 95 runs with Foster before Simon Kerrigan ended his stay at the crease, while Foster hit 82 runs before he also fell victim to Kerrigan shortly before the light failed.
Kent's Calum Haggett bagged three wickets for 38 runs to ensure the hosts made the most of a rain-interrupted opening day against Glamorgan.
Haggett accounted for openers Ben Wright (five) and Will Bragg (six), and also removed Marcus North for 46 as the visitors ended the day on 155 for four.
Strugglers Hampshire and Gloucestershire were left frustrated as heavy rain wiped out the first day at the Ageas Bowl.
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- James Middlebrook