Complex sentence: The Local Government Act 1888 adjusted the county boundaries in 1889, so that the entire district was in Hertfordshire; forming part of a long, thin protrusion into Middlesex surrounded by that county on two sides, to the north and south.
Split into simpler sentences: The Local Government Act 1888 adjusted the county boundaries in 1889, so that the entire district was in Hertfordshire. The district formed part of a long, thin protrusion into Middlesex and was surrounded by that county on two sides; to the north and south.