We can still distinguish some of the first steps in scientific knowledge. Thus, counting began with calculations on one's fingers, a method still familiar to children. Finger counting explains the origin of the decimal system. The simplest, and probably the earliest, measures of length are those based on various parts of the body. Some of our Indian tribes, for instance, employed the double arm's length, the single arm's length, the hand width, and the finger width. Old English standards, such as the span, the ell, and the hand, go back to this very obvious method of measuring on the body. CALCULATION OF TIME; THE CALENDAR It is interesting to trace the beginnings of time reckoning and of that most important institution, the calendar. Most primitive tribes reckon time by the lunar month, the interval between two new moons (about twenty- nine days, twelve hours). Twelve lunar months give us the lunar year of about three hundred and fifty-four days. In order to adapt such a year to the different seasons, the practice arose of inserting a thirteenth month from time to time. Such awkward calendars were used in antiquity by the Babylonians, Jews, and Greeks; in modern times by the Arabs and Chinese. The Egyptians were the only people in the Old World to frame a solar year. From the Egyptians it has come down, through the Romans, to us. [13] [Illustration: STONEHENGE On Salisbury Plain in the south of England: appears to date from the close of the New Stone Age or the beginning of the Bronze Age. The outer circle measures 300 feet in circumference; the inner circle, 106 feet. The tallest stones reach 25 feet in height. This monument was probably a tomb, or group of tombs, of prehistoric chieftains.] EARLY DRAWING AND PAINTING The study of prehistoric art takes us back to the early Stone Age. The men of that age in western Europe lived among animals such as the mammoth, cave bear, and woolly-haired rhinoceros, which have since disappeared, and among many others, such as the lion and hippopotamus, which now exist only in warmer climates. Armed with clubs, flint axes, and horn daggers, primitive hunters killed these fierce beasts and on fragments of their bones, or on cavern walls, drew pictures of them. Some of these earliest works of art are remarkably lifelike. [Illustration: HEAD OF A GIRL (Musée S. Germain, Paris) A small head of a young girl carved from mammoth ivory. Found at Brassempouy, France, in cave deposits belonging to the early Stone Age. The hair is arranged somewhat after the early Egyptian fashion. Of the features the mouth alone is wanting.] [Illustration: PREHISTORIC ART SKETCH OF MAMMOTH ON A TUSK FOUND IN A CAVE IN FRANCE CAVE BEAR DRAWN ON A PEBBLE BISON PAINTED ON THE WALL OF A CAVE WILD HORSE ON THE WALL OF A CAVE IN SPAIN.