Man's physical universe

xanabras

122 THK UNIVERSE A VAST SYSTEM OF PARTS

where the dial is erected. This change in the design of the sundial

corrects for the apparent movement of the sun in a north and south

direction.

Mechanical Devices. The need arose for an instrument that

would measure time both day and night, independently of the

sunlight.

1. Tapers graduated into equal parts were once used to indicate

time, inasmuch as they burned at a roughly constant rate.

2. The clepsydra was a simple measuring device which depended

upon the time required for a given amount of water to trickle through

a hole in the bottom of a suitable container. The modern hourglass

or sandglass is a variation of the clepsydra.

3. More complicated deinces depended upon the motion of a pointer

connected by gears to a float that was raised as the vessel in which it

rested was gradually filled with water.

4. Power-driven clocks superseded the water clocks because they

removed difficulties involved in maintaining a water supply that would

not freeze in the winter and because water clocks could not be used

at sea. (In navigation, longitude can be determined 07ily if one knows

the time somewhere else.)

5. The modern electric clocks, which are rapidly replacing other types

of clocks, are run by the alternating current whose pulsations are

checked by master clocks at the powder plants. In reality, then, all

electric clocks are merely devices to receive and indicate time signals

transmitted by alternating current from a master clock. Most power

companies generate 60-cycle current, that is, current which alternates

exactly 120 times a second. Inside an electric clock is a tiny motor

synchronized with the generators in the powerhouses.

The Calendar.

Julius Caesar in 45 B.C. introduced the Julian calendar invented by

Sosigenes. He used 3653^ days as the year and invented the leap year

to take care of the quarter-days. The Julian year is too long by 11

minutes and 14 seconds. In 400 years, this variation w^ould amount

to 3 days. In the sixteenth century, this error had mounted to 1 1 days.

Pope Gregory, at that time, dropped 10 days from the calendar, but not

without considerable opposition.

The Gregorian calendar provides that century years are not leap

years unless they are divisible by 400, thus making another such radical

revision of the calendar unnecessary, because our present year is too

long by only 26 seconds. Thus the Gregorian calendar provides for

fewer leap years than the Julian calendar.

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