at the entrance to a office in lond

at the entrance to a office in london, there was a book which all employees had to sign when they arrived each morning. at nine o'clock, the manager's secretary, who lived in a small flat above the office, had to draw a red line under the last name in the book, and anyone who came after that to explain why he was late. whenever there was a thick fog in the city,the first person to arrive late usually wrote delayed by fog under the red line in the book, and then everybody else who came after that just put ditto underneath. but one foggy morning, the first man to arrive late wrote my wife had a baby early this morning instead of delayed by fog under the red line in the book. twenty or thirty people who came after him put ditto underneath this as usual.
please answer these questions
1. why did the employees have to sign the book?
2. what was a common reason for being late?
3. what was the first man's reason on the day this story deals with?
4. what meaning did the rest of the employees really intend when they wrote ditto that morning?
5. what did their ditto seem to mean?