Standard time at Boston is 16 min utes earlier than mean solar local time; at New York, 4 minutes ear lier; at Washington, 8 minutes faster; at Charleston, 19 minutes faster; at Detroit, 28 minutes slower; at Kan sas City, 18 minutes faster; at Chi cago, 10 minutes slower; at St. Louis, 1 minute faster; at Salt Lake City, 28 minutes faster; at San Francisco, 10 minutes faster.
This difference between standard and mean solar local time causes some inconvenience. At some points, sunrise, noon, or sunset may vary from standard time as much as half an hour. The variation, however, cannot be greater than this, and in some localities is very slight. This difference may affect such matters as the hour for meals and the time given in the almanac (which is based on mean solar local time) as the hour of sunrise or sunset. Proper adj ust ments can be made, however, by esti mating the difference in time be tween the meridian of the place and the standard meridian at the rate of four minutes to each degree of longitude. Standard time is now in use practically throughout the civil ized world. The difference in time between two places can be easily de termined by observing on a map their distance apart in degrees of longitude and allowing four minutes to each degree based on the following calcu lation: 1440 minutes 1 day, or revolution of the earth.
I revolution of the earth is 360 degrees. Therefore, 1 degree 4 minutes.
The following is a table of the standard for the measurement of time: 60 seconds 1 minute.
60 minutes 1 hour.
24 hours 1 day.
7 days 1 week.
4 weeks 1 lunar month.
28 days 1 lunar month.
30 days (in computing inter est) 1 month.
28, 29, 30, or 31 days 1 cal. month.
12 calendar months 1 year.
131un. filo., 1 day, 6 hours 1 Julian Year.
365 days 1 com. year.
366 days 1 leap year.
365} days 1 Julian year.
52 weeks and 1 day (12 cal.
mo.) 1 year.
365 d., 5 h., 48 m., 49 s 1 solar year.
365 d., 6 h., 9 m.,12 s I sidereal year.
100 solar years 1 century.
The Calendar.—The following jin gle will be found convenient to recall the variations in the calendar months: Thirty days bath September, April, June, and November; All the rest have thirty-one Except the second month alone Which has but twenty-eight, in fine, 'Til leap year gives it twenty-nine.
The common year has 365 days, or 52 weeks and 1 day; the leap year 366 days.
The length of the solar year 1880 was 365 da., 5 hr., 48 47i sec., which is nearly one quarter of a day longer than the common year. On the supposition that 365-Is days was the true solar year, Julius Cmsar in troduced a calendar in which every year exactly divisible by 4 (every fourth year) included 366 days, called leap year. The error of the Julian calendar amounts to 3.1142 days in four centuries. To correct the error, Pope Gregory XIII, in 1582, modified the Julian calendar by making three of the centennial years in every four centuries common years and one a leap year. Hence every
year that is divisible by 4 and is not a centennial year is a leap year, and every centennial year that is divis able by 400 is a leap year. The year 1900 will be a common year, and the year 2000 a leap year.
Pendulnm.—Any weight suspended so as to swing freely under the action of gravity is called a pendulum. Sci entific investigation of the swing of the pendulum has proved that the time of the vibration of the pendu lum is in proportion to the square root of its length; that is, a pendu lum which vibrates once in a second must be four times as long as one which vibrates once in half a second, sixteen times as long as one which vi brates in one fourth of a second, and so on. It has been further shown that the time of the vibration is inde pendent of the length of the arc, or the distance through which the pen dulum swings, so long as the arc is comparatively small. This is what gave rise to the application of the pendulum to the regulation of time pieces.
The attraction of gravity which causes the vibration of the pendulum depends upon the distance of a body from the center of the earth. The earth being flattened at the poles, this distance is greater at the equator and less at the poles. Hence the same pendulum will vibrate faster at the poles than at the equator. Hence the pendulum is used to determine the force of gravity at various points on the earth's surface. And the length of a pendulum vibrating sec onds under stated conditions is now the accepted standard of length in Great Britain and the United States. In the vicinity of New York a pendu lum vibrating once in a second is 3911/4 inches long.
To shorten the pendulum of a clock makes it run faster; to lengthen it makes it run slower.
Measuring Temperature.—We com monly use a thermometer known as Fahrenheit's.
In scientific work the FAHREN- Cmimx centigrade (a word HEIT GRADE meaning " 100 degrees ") is used. Because there water are just 100° between Boils 212° — 100° the freezing and boiling points of water, on the Blood 0 centigrade thermometer, and the freezing point Heat 98 — is at 0°, that would be easier than the Fahren- Water heit to work with if we Freezes 32°— 0° were used to it. 0°— Specific Gravity. — The specific gravity of any substance is the num ber of times its weight eontains the weight of an equal volume of water. Since the weight of a cubic foot of water at its greatest density is 1,000 oz., or 62i lb., the weight of a cubic foot of any substance is found by multiplying 1,000 oz. avoirdupois by its specific gravity.