MECHANICAL PHILOSOPHY. STATICS. [UNITS OTLKNOTII A*n TIMI. 



We must, therefore, liave tome standard measure of 



$NIT OF LENGTH. Tho Act of Parliament just 

 cited hat also determined the standard unit of the 

 measure of length. By the fint section of that act, it 

 is enacted" That from and after the 1st day of May, 

 1895, the straight line, or distance between the centres 

 of the two i.iiits in the gold studs in the straight brass 

 rod, now in the custody of the clerk of the House of 

 Commons, whereon the words and figures, 'Standard 

 yard, 1760,' are engraved, shall be, and the same is hereby 

 declared to be, the original and genuine standard of that 

 measure of length or lineal extension called a yard, and 

 that the same straight line or distance between the 

 centres of the said two points in the said gold studs in 

 the said bran rod, the brass being at the temperature of 

 62 degrees by Fahrenheit's thermometer, shall bo and 

 is hereby denominated the 'Imperial Standard Yard,' 

 and shall be, and is hereby declare*! to be, the unit or 

 only standard measure of extension, wherefrom or 

 whereby all other measures of extension whatsoever, 

 whether the same be lineal, superficial, or solid, shall be 

 derived, computed, or ascertained ; and that all measures 

 of length shall be taken in parts or multiples, or certain 

 proportions of the said standard yard ; and that one- 

 third part of the said standard yard shall be a foot, and 

 the twelfth part of such foot shall be an inch, and that 

 the pole or perch in length shall contain 5 such yards 

 and a half, the furlong 220 such yards, and the mile 

 1,700 such yards. 



"And whereas the said standard yard, if lost, de- 

 stroyed, defaced, or otherwise injured, should be re- 

 stored of the same length by reference to some invariable 

 natural standard ; and whereas it has been ascertained, 

 by the commissioners appointed by his Majesty to in- 

 quire into the subject of weights and measures, that the 

 said yard hereby declared to be the Imperial Standard 

 Yard, when compared with a pendulum vibrating seconds 

 of mean time in the latitude of London, in a vacuum, at 

 the level of the sea, is in the proportion of 36 to 39 

 inches, and 1,393 ten thousandths part of an inch : Be 

 it therefore enacted and declared, That if at any time 

 hereafter the said imperial standard yard shall be in any 

 manner destroyed, defaced, or otherwise injured, it 

 shall and may be restored by making, under the 

 direction of the Lord High Treasurer, or the Commis- 

 sioners of His Majesty's Treasury of the United 

 Kingdom of Great Britain and Ireland, or any three of 

 them, for the time being, a new standard yard bearing 

 the same proportion to such pendulum as aforesaid, as 

 the said imperial standard yard bears to such pendulum." 



UNIT OF TIME. Time is the ultimate natural 

 standard to which all measures of length, weight, and 

 capacity are referred in this kingdom. Time certainly 

 is one of the most convenient, and perhaps the only 

 natural standard to which we can have recourse, to fix 

 with scientific accuracy our units of measurement. The 

 value of time in this respect depends upon tho fact 

 deduced by astronomers, from observations collected 

 and registered for many centuries, that the mean length 

 of day and night is invariable ; in other words, that the 

 earth always completes its rotation on its axis in the 

 same period of time. This portion of time is obtained 

 by observing the period which elapses between the 

 passage of a fixed star over a certain imaginary line in 

 the heavens, called tho meridian of the place of obser- 

 vation, and its next appearance on this lino, and is 

 called a sidereal day. The sidereal day is divided into 

 twenty-four hours, each hour into sixty minutes, and 

 each minute into sixty seconds. A clock is an instrument 

 for measuring time, and is set in motion by a weight ; 

 its motion is regulated by tho vibrations of a pendulum, 

 and the number of its vibrations are registered by means 

 of a series of wheels, and indicated on the graduated 

 face of the clock by the motion of hands or pointers. 

 The length of the pendulum is so regulated that its tirno 

 of vibration may bo as nearly as possible one second ; and 

 these vibrations being registered by tho clock, if tho 

 clock indicate that 24 hours, or 24 x CO x GO, or 80,400 



seconds have elapsed between one transit of tho star 

 over the meridian and another, tho clock is said to be 

 correct, and will afford us an accurate measure of sidereal 

 time. Such a clock is called a sidereal clock, and is one 

 of the most valuable and useful instruments in an ob- 

 servatory. Sidereal time is not, however, the time used 

 for the ordinary transactions of life. Tho true solar day 

 is tin- interval which elapses between ono passage of the 

 sun over the meridian and another, as shown by a dial 

 or other means of astronomical observation ; this day is 

 not invariable in length, but changes from day to day, 

 being sometimes longer and sometimes shorter. To avoid 

 this inconvenience, a mean solar day is chosen by tho 

 supposed revolution of a fictitious sun, which shall be 

 invariable in length ; and this time is divided into 24 

 hours, and these hours into minutes and seconds, as in 

 tho case of the sidereal day. This mean solar day is the 

 time used for the ordinary or civil reckoning of tinif. 

 What is called the equation of time, is an astronomical 

 calculation, which gives the difference between the time 

 shown by the dial or tho true solar time, and that in- 

 dicated by the ordinary clocks, which show the mean 

 solar or civil measure of time. The sidereal day is 23 hours, 

 56 minutes, 4 seconds, and nine hundredths of a second 

 of a mean solar day ; and the mean solar day is 24 hours, 

 3 minutes, 56 seconds, and 55 hundredth parts of a 

 second of a sidereal day. The pendulum vibrating 

 seconds, from which, by Act of Parliament, the standard 

 measure of length, if lost, is to be obtained, is to be a 

 pendulum vibrating a second of mean solar time under 

 the conditions specified in the act. So great, however, 

 are the scientific and mechanical difficulties to be over- 

 come in determining accurately the length of this second's 

 pendulum, that though the standard yard was so injured 

 by the firo when the houses of Parliament were burnt 

 down in 1834, and the standard pound troy altogether 

 lost, no attempt has been made to restore the lost 

 standards. The last scientific commission seems to have 

 considered the attempt to do so altogether hopeless. 

 Fortunately, the Royal Astronomical Society had a very 

 beautiful scale constructed about the year 1832, and 

 three feet of this scale were compared many hundreds 

 of times with the Parliamentary standard ; and this scale 

 must now be considered the scientific English standard, 

 and the best evidence of the parliamentary standard 

 which exists. There is also great doubt as to the ac- 

 curacy of Captain Hater's determination of the length 

 of the second's pendulum which is adopted by the Act 

 of Parliament ; so that if the standard were to be re- 

 stored according to that act, it would probably differ 

 from tho lost standard (Bailey's Report on a Standard 

 Scale, Astronom. Soc. Mem. voL ix.) Two Acte of 

 Parliament, 5 and 6 Wm. IV., c. 63, and 1C Vic., c. 2!>, 

 June, 1853, have been enacted since tho fire, on tho 

 subject of weights and measures ; but neither of them 

 notice tho loss of the standards, speaking of them as if 

 they were still in existence. 



FRENCH STANDARDS. The French standards arc 

 derived from actual measurements of the earth's surface. 

 From these measurements the length of a lino drawn 

 from the pole of tho earth to the equator is dedur. .1. 

 The ten-millionth part of this line is called a metro ; 

 and this is the French standard of length. Tho deca- 

 metre is 10 metres, the hecto-metre is 100 metres, the 

 kilo-metro 1,000 metres, and tho myria-metre is 10,000 ; 

 while the deci-motre is the 10th part of a metre, tho 

 centi-metre the 100th part of a metre, and tho milli- 

 metre tho 1,000th part of a metre Greek prefixes being 

 used for the multiples of tho metre, and Latin for its 

 parts. The gramme, which is the standard of weight, is 

 derived from the standard of length, and is equal to tho 

 weight of a cubic centi-metro of distilled water, weighed 

 at tho freezing point. Tho same prefixes are used 

 before the gramme, which are added to the metre to 

 express its multiples and parts. Thus a kilo-gramme is 

 a 1,000 grammes, and a milli-grammc the 1,000th part 

 of a gramme. 



The metro is equal to 3-280899 English feet, and tho 

 kilo-gramme is equal to 3'2045!>7 pounds avoirdupois. 



