Complex sentence: The 1861 census describes Thomas as a tool manufacturer employing 95 men and 30 boys; he had 200 men working for him at the time of the 1871 census and 300 by that of 1881.
Split into simpler sentences: The 1861 census describes Thomas as a tool manufacturer employing 95 men and 30 boys. By the 1871 census he was employing 200 men and 300 by that of 1881.