| 摘要 |
<p>The perpetual calendar is formed of four concentric, coloured, discs. The principles are based on the fact that a given year is characterised by the week day of its 1st January and whether or not it is a leap year. These two features are in one circular disc, together. When used, they give immediately details of the days of the week of each month of the year concerned. This disc is printed on both sides. The other discs carry initials for days of the week and figures 1 to 31.</p> |
