Both these dials were in use about the beginning of our era and were covered by the great eruption of Vesuvius in 79 A.D., which destroyed Pompeii and Herculaneum.

Modern sundials differ only in being more accurately made and a few “curiosity” dials added. The necessity for time during the night, as man's life became a little more complicated, necessitated the invention of time machines. The “clepsydra,” or water clock, was probably the first. A French writer has dug up some old records putting it back to Hoang-ti 2679 B.C., but it appears to have been certainly in use in China in 1100 B.C., so we will be satisfied with that date. In presenting a subject to the young student it is sometimes advisable to use round numbers to give a simple comprehension and then leave him to find the overlapping of dates and methods as he advances. Keeping this in mind, the following table may be used to give an elementary hint of the three great steps in time measuring:

I have pushed the gear wheel clocks and watches forward to 2000 A.D., as they may last to that time, but I have no doubt we will supersede them. At the present time science is just about ready to say that a time measurer consisting of wheels and pinions—a driving power and a regulator in the form of a pendulum or balance, is a clumsy contrivance and that we ought to do better very soon; but more on this hoped-for, fourth method when we reach the consideration of the motion on which we base all our time keeping.

It is remarkable how few are aware that the simplest form of sundial is the best, and that, as a regulator of our present clocks, it is good within one or two minutes. No one need be without a “noon-mark” sundial; that is, every one may have the best of all dials. Take a post or any straight object standing “plumb,” or best of all the corner of a building as in [Fig. 3]. In the case of the post, or tree trunk, a stone (shown in solid black) may be set in the ground; but for the building a line may often be cut across a flagstone of the footpath. Many methods may be employed to get this noon mark, which is simply a north and south line. Viewing the pole star, using a compass (if the local variation is known) or the old method of finding the time at which the shadow of a pole is shortest. But the best practical way in this day is to use a watch set to local time and make the mark at 12 o'clock.

[LOI]

Fig. 3—Noon-Mark Sundials

On four days of the year the sun is right and your mark may be set at 12 on these days, but you may use an almanac and look in the column marked “mean time at noon” or “sun on meridian.” For example, suppose on the bright day when you are ready to place your noon mark you read in this column 11:50, then when your watch shows 11:50 make your noon mark to the shadow and it will be right for all time to come. Owing to the fact that there are not an even number of days in a year, it follows that on any given yearly date at noon the earth is not at the same place in its elliptical orbit and the correction of this by the leap years causes the equation table to vary in periods of four years. The centennial leap years cause another variation of 400 years, etc., but these variations are less than the error in reading a dial.

SUN ON NOON MARK, 1909
DateClock
Time		DateClock
Time		DateClock
Time
Jan. 212:04May 111:57Sep. 3011:50
“ 412:05 “ 1511:56Oct. 311:49
“ 712:06 “ 2811:57 “ 611:48
“ 912:07June 411:58 “ 1011:47
“ 1112:08 “ 1011:59 “ 1411:46
“ 1412:09 “ 1412:00 “ 1911:45
“ 1712:10 “ 1912:01 “ 2611:44
“ 2012:11 “ 2412:02Nov. 1711:45
“ 2312:12 “ 2912:03 “ 2211:46
“ 2812:13July 412:04 “ 2511:47
Feb. 312:14 “ 1012:05 “ 2911:48
“ 2612:13 “ 1912:06Dec. 111:49
Mar. 312:12Aug. 1112:05 “ 411:50
“ 812:11 “ 1612:04 “ 611:51
“ 1112:10 “ 2112:03 “ 911:52
“ 1512:09 “ 2512:02 “ 1111:53
“ 1812:08 “ 2812:01 “ 1311:54
“ 2212:07 “ 3112:00 “ 1511:55
“ 2512:06Sep. 411:59 “ 1711:56
“ 2812:05 “ 711:58 “ 1911:57
Apr. 112:04 “ 1011:57 “ 2111:58
“ 412:03 “ 1211:56 “ 2311:59
“ 712:02 “ 1511:55 “ 2512:00
“ 1112:01 “ 1811:54 “ 2712:01
“ 1512:00 “ 2111:53 “ 2912:02
“ 1911:59 “ 2411:52 “ 3112:03
“ 2411:58 “ 2711:51
The above table shows the variation of thesun from “mean” or clock time, by even minutes.