837 



WEIGHTS AND MEASURES, STANDARD. 



WEIGHTS AND MEASURES, STANDARD. 



838 



thermometer The act goes on in many words to say that the 



pendulum vibrating seconds of mean time in the latitude of London * 

 in a vacuum at the level of the sea is 39'1393 inches of the said 

 standard. 



2. The standard brass weight of one pound troy weight, made in the 

 year 1758, now in the custody of the clerk of the House of Commons, 

 shall be the original and genuine standard measure of weight . . . 

 The act goes on to say that the cubic inch of distilled water, weighed 

 in air by brass weights, at 62 of Fahr., the barometer being at 30 

 inches, is equal to 252'458 grains. 



It happened fortunately for the scientific standard, that about the 

 year 1832 the council of the Royal Astronomical Society caused a 

 scale to be constructed for themselves, and obtained permission of the 

 Speaker of the House of Commons to compare it with Bird's two 

 standards, which was done in the beginning of 1834, by a much more 

 extensive set of experiments than had ever been made before for a 

 like purpose, conducted chiefly by Mr. Baily and the late Lieutenant 

 Murphy. This is now, in fact, the standard scale of the country ; or, 

 at least, the only measure from which the standard scale can be 

 deduced. The manner of conducting the comparisons has already 

 been slightly described ; we shall now proceed (from the Report already 

 quoted) to give some account of the difficulties which were found in 

 the way of measurement, and of the results. 



This scale is a cylindrical tube of brass 63 inches long, 112 inches 

 and "74 inches in exterior and interior diameter. Three thermometers 

 are imrnoveably inserted into its length, and the ends are stopped by 

 brass plugs. Two parallel lines (-09 of an inch apart) are drawn in the 

 upper surface; and, commencing H inches from one end, at the 

 distance of every foot, a palladium pin is inserted in the tube, between 

 those lines ; on each of which pins, at proper distances, a fine line is 

 cut to designate the length of a foot. The first foot is similarly 

 divided into inches and tenths ; and the middle foot (there being five 

 in all) is bisected. The three middle feet constituted the yard which 

 wag used in the comparisons. It was found that any constraint, 

 , however slight, affected the expansion and contraction of the bar; 

 even the friction arising from its first supports, which were lined with 

 baize : it was therefore found necessary to support it, when under the 

 microscopes, on friction-rollers : and care was taken that these should 

 always be placed under the same points of the tube. To give an idea 

 of the power of the mode of comparison, it was found, by fourteen 

 experiments agreeing very well with each other, that the middle yard 

 was shortened '48 of one of the divisions of the micrometer-head 

 (described at the beginning of the article), or '000024 of an inch, by 

 nothing but removing the plugs from the end of the tube. 



Nothing can be known of such a bar as a scientific standard until 

 the rate at which it expands by the action of heat is determined. By 



i of six experiments, taken with the tube at the freezing and 

 boiling temperatures, it was found that every addition of 1 of Fahr. 

 to the temperature lengthened the centre yard by '000377 of an inch, 

 or 7't> divisions of the micrometer-head. 



The instrument being placed ready for observation, and two scales 

 being put down for comparison, one observer may bring both the 

 micrometers to the ends of one scale, or one observer may be placed at 

 one end, and another at the other. In the latter case, a new cause of 

 error enters, of which it is impossible to give any account, though a 

 remedy may be provided. It is not true that two persons, though 

 using exactly the same instrument, and noting the same phenomenon 

 under the same circumstances, will note it exactly in the same way. 

 When one observer made (as he thought) the coincidence of the inter- 

 section of the micrometer-wires with the dot or line at the end of a 

 scale, another, looking into the microscope, would seldom or never 

 agree with the former that the coincidence was exactly made, but 

 would turn the micrometer-head three or four divisions, one way or the 

 other, before he (the second) could be satisfied that the coincidence 

 was perfect. This difference of the manner of observing, arising from 

 the peculiar habits of vision and judgment of the observers themselves, 

 has received the name of the personal equation, and its amount, as 

 between any pair of observers, can be ascertained by experiment. If 

 one observer made the coincidences at both ends, it would matter 

 nothing what his manner of observing was, since, however much he 

 might differ from absolute correctness (be that what it may), he would 

 differ by the same amount at both ends, and the length of the scale 

 would not be affected. If, when two observers are employed, they 

 make a given number of comparisons, and then change places and make 

 the same number, the mean of all their observations will be unaffected 

 by their mode of observing, since, if the scale be made too long in the 

 first set, it will be made as much too short in the other, and vice 

 versa. [EQUATION, PERSONAL.] Some of the personal equation might 

 arise from the curious figures which the dots of the old scales (into 

 which beam-compasses had been inserted) presented when viewed 

 under the microscope. Bird's standard of 1758, for instance, had 

 pear-shaped holes at its extremities, the centres of which no two 

 persons could agree upon. 



The following results will give a notion of the degree of accuracy 

 obtained hi the workmanship of scales. - The Astronomical Society's 



* The latitude of London was rather a vague phrase for legislation which 

 could not let " length " pass without the explanation " lineal extension." 



scale was compared with the imperial standard (Bird's of 1760) ; the 

 Royal Society's scale of 1742, having two scales in it marked E and 

 Exch. ; a scale called Aubert's, the prototype of one which was used 

 in the Indian survey by Lambton; one which had been used by 

 Sir G. Shuckburgh; one belonging to the town of Aberdeen; one 

 belonging to Mr. T. Jones ; and four new ones made after the model 

 of the Society's scale, one for the Danish government, one for the 

 Russian government, one retained for himself by Mr. Simms the con- 

 structor, and one for Mr. Baily. Calling the middle yard of the 

 Astronomical Society's scale 36 inches, the different scales are as 

 follows, each from the mean of many observations : 



Temperature is not here alluded to, it being presumed, of course 

 that the effect of temperature upon the difference of two scales is 

 inappreciable : thus the Astronomical Society's standard being '000376 

 longer than the imperial standard, and the standard temperature 

 being 62 , the length of the former standard, observed at 62 and 

 diminished by -000376 of an inch, will give the true standard of 

 the law. 



It is believed, after all, that the imperial standard is about l-140th 

 of an inch longer than the old standard of the country but this 

 matters nothing to the scientific part of the question, for all the scales 

 which have been used in trigonometrical surveys have now been dili- 

 gently compared with the Astronomical Society's scale, and are there- 

 fore known, independently of the national standard, as long as the 

 latter scale exists. The only thing to.be feared is the loss of this 

 last-mentioned standard ; the government might keep it, but cannot 

 be trusted to use it ; the Society, which knows how to use it has no 

 place of perfect security in which to keep it. 



We shall not here enter into the various modes used by Sir G 

 Shuckburgh, and subsequently by Captain Kater, for the determi- 

 nation of the standard of weight. An old standard pound exists in 

 the Exchequer, from which in 1758 a copy was made for the com- 

 uttee of the House of Commons. This last, as we have seen has 

 xsen declared the standard, and was never recovered from the ruins of 

 the late House of Commons. The original standard of weight, as 

 prescribed in a statute of 51 Hen. III., called Amza. Panis et oSrvMa,, 

 was that an English penny called the sterling, round without clipping 

 should weigh 32 grains of wheat, well dried and gathered out of the 

 middle of the ear; and that 20 pence should make an ounce and 

 12 ounces a pound.* 



In 1838 a Treasury Commission was appointed to consider the best 

 >f replacing the ruined standards of weight and measure The 

 report of the commission was presented to the government on the 

 1st of December, 1841. The opinion was expressed therein that the 

 definition contained in the act 5 Geo. IV. cap. 74, by which the 

 standard yard was declared to be a certain brass rod was the best 

 which could possibly be adopted. With regard to the restoration of 

 the standard the commissioners were not prepared to recommend con- 

 formably to the said act, that it should be effected by taking the length 

 which shall bear a certain proportion to the length of the pendulum 

 vibrating seconds of mean time, in the latitude of London, in a vacuum 

 at the level of the sea. They remarked that since the passing of the 

 act above mentioned, it had been ascertained that several elements of 

 reduction of the pendulum experiments referred to therein were 

 doubtful or erroneous. Dr. Young had shown that the reduction to 

 the level of the sea was doubtful. Bessel and Baily had proved that 

 the reduction for the weight of the air was erroneous : Baily had pointed 

 out that the specific gravity of the pendulum was erroneously estimated 

 and that the defects of the agate planes introduced some degree of 

 doubt. * mally, Kater had shown that very sensible errors were T intro- 

 duced m the operation of comparing the length of the pendulum with 

 Shuckburgh s scale used as the representative of the legal standard. 

 On these grounds it appeared evident to the commissioners that the 

 course prescribed by the act would not necessarily reproduce the length 

 of the original yard. They remarked, however, that several measures 

 were in existence which had been accurately compared with the former 



Here ends the article on Standard Weights and Measures as it originally 

 ippeared in the ' Penny Cyclopaedia ' from the pen of the late Mr. Sheepshanks 

 he continuation is mainly derived from a paper by Mr. Airy, published in the 

 ' Philosophical Transactions' of the Royal Society for 1857 ' Account of the 

 Construction of the New National Standard of Length, and of its nnnciiml 

 Copies,' vol. clxvii., pp. 621-702. 



