Whitehurst & Son sundial (1812)
Significantly, on the plate we can see a pair of scales that help the observer make the equation of time correction. One scale gives the date in months and days. Along side of it, another scale is engraved with the minutes on that day that the watch would be running faster or slower. Here it is labeled "Watch Slower, Watch Faster. The 15th April is one day when no conversion needs to be made. This dial can be used both to read solar time shown by sundials and also the mean time that is favoured by clocks, with the practical purpose that observers can use the dial to calibrate their pocket watches, which in 1812 would not always run true. By 1820 watches manufacture had improved and the Lever escapement had become popular, so frequent calibration was no longer needed.