654 THE POPULAR SCIAWCE MONTHLY, 



true length for that temperature. Is has lately been shown, however, 

 by the experiments of Professor Rogers, that if two steel bars, one of 

 which is nickel-plated, be subjected to a gradual change of tempera- 

 ture, they will acquire their true length after the temperature has been 

 maintained constant for about twelve hours ; but, if the change be an 

 abrupt one, it is not safe to compare them until after the lapse of 

 from forty-eight to sixty hours. 



Enough has been said to indicate what great precautions must be 

 taken in order to obtain accurate copies of a given standard of length. 

 AVe may now consider how the standard measures at present in use 

 were originally obtained, and how they are related to each other. 

 We will confine our attention to the measures of France and Eng- 

 land, since these possess more interest for us than do the measures of 

 other nations, with which we are less familiar. 



It is quite generally supposed that the length of a yard exactly cor- 

 responds to the length of a pendulum beating seconds of time, in a 

 vacuum, in the latitude of London, at the sea-level. This, however, 

 is not the fact. The act of Parliament in relation to this matter has 

 been generally misunderstood, for it does not declare the length of the 

 yard to be absolutely that of the j)endulum ; in truth, these lengths 

 are not the same. Parliament only provided that, in case the original 

 standard should be lost, it could be restored by reference to the unit- 

 pendulum. The standard that was legalized was made by Bird, from 

 Graham's scale, in the year 17G0. It was named the " imperial stand- 

 ard yard." . 



According to experiments conducted at that time, it was found 

 that the relation between the length of the standard imperial yard 

 and that of a seconds-pendulum w^as in the proportion of thirty-six 

 inches to thirty-nine inches and -^^^^-^ of an inch. On October 16, 

 1834, both Houses of Parliament were destroyed by fire, and, although 

 the imperial yard was found in the ruins, it had become unfit for use 

 as a standard. The problem of its restoration w^as then presented, but 

 since the passage of the act of 1824, which declared the relations be- 

 tween the pendulum and the lost standard, it had been found that the 

 data from which the relations were calculated were, in several respects, 

 unreliable. It was finally decided not to attempt the restoration of 

 the lost standard by means of the pendulum, but to work from the 

 various standards which had been compared with it. For this pur- 

 pose six different scales were found available, among which was 

 the tubular scale belonging to the Royal Astronomical Society ; 

 but this scale was not the principal authority from which the new 

 standard was constructed, although it is so asserted in both Apple- 

 tons' and Johnson's Cyclopaedias. The scales actually made use of 

 were two by Shuckburgh, one by Kater, that belonged to the Royal 

 Society, and two bars of the Ordnance Department. The work of 

 renewing the standard was intrusted to Sir Francis Baily, but he died 



