I was idly trolling through Wikipedia's Did you know . . . section the other day and saw an article on sundial cannon. Knowing precisely when noon is becomes really important in oceanic navigation as Dava Sobel explained in her book Longitude [prev]. Navigating officers use their sextants to determine the moment when the sun stops climbing higher in the sky and starts its afternoon descent to the horizon. If you know when Noon was (or will be) that day back home in Greenwich (because you've got a super accurate, motion-proof, mechanical clock on board) it is a simple matter - and a lot of logarithms - of using spherical geometry to calculate how far you are across the face of the globe . . . and how close you are to a dangerous lee-shore. The lives of pre-GPS sailors depended upon this knowledge. Landlubbers also like to know when noon, and dinner-time, is, especially if they don't have a bio-clock like my father's. He needed his lunch at 1 o'clock +/- 2 minutes, or his sky fell.
Here's one working. If you're a boy you'll be off for a while down the you-tube side bar watching chaps firing golf-falls across car-parks and otherwise making a lot of noise. When I was a kid we had a teeny-tiny spring loaded cannon that could fire match-sticks at ranks of enemy toy-soldiers. That was a less mind-corrupting sport than say Grand Theft Auto which engages modern boys with the same murderous intent. A palomino was disconcerted to hear one of her barely teenaged sons explaining to a pal "Naa, you have to throw the whore out of the car-door as you go round the corner . . .".
A sun-dial lens can also be used to tally up the hours of direct sun-light in the day if you have the focused light travel over a sheet of paper. If it's cloudy the paper will be white, if sunny it will be charred. If properly calibrated, you can add up when and for how long the sun shone. It's a bit like a barograph which instrument causes an ink-filled pen to move up and down according to the air-pressure. Needless to say, my nautical father had one of these and one his rituals was periodically to change the paper on the clock-work recording drum. Blimey, I've made the Old man out to be thoroughly OCD but I don't this is true; nor did he dress us up in sailor suits and pipe him aboard with a bo'sun's whistle when he came home.