May 1 8, 1876J 



NATURE 



57 



action at a distance, was the result of minute and accurate 

 measurement of electric forces. 



Joule's discovery of thermo-dynamic law, through the 

 regions of electro-chemistry, electro-magnetism, and 

 elasticity of gases was based on a delicacy of thermo- 

 metry which seemed impossible to some of the most 

 distinguished chemists of the day. 



Andrews's discovery of the continuity between the 

 gaseous and liquid states was worked out by many years 

 of laborious and minute measurement of phenomena 

 scarcely sensible to the naked eye. 



Here, then, we have a very full recognition of the im- 

 port-mce of accurate measurement, by one who has a 

 perfect right to speak authoritatively on such a subject. 

 It may indeed be maintained that no accurate knowledge 

 of any thing or any law in nature is possible, unless we 

 possess a faculty of referring our results to some unit of 

 measure, and that it might truly be said — to know is to 

 measure. 



To resort to a homely illustration of this proposition? 

 let us suppose a traveller in the unknown wilds of the 

 interior of Africa, observing before him a number of ele- 

 vations of the ground, not differing materially from one 

 another in apparent magnitude. Without measuring ap- 

 paratus the traveller could form no conclusion regarding 

 the geographical importance of those visible objects, 

 which might be mere hillocks at a moderate distance, or 

 the domes of an elevated mountain range. In stepping 

 his base line, however, and mounting his distance- 

 measurer, he soon ascertains his distances and observa- 

 tions with the sextant and compass, give the angles of 

 elevation and position of the objects. He now knows 

 that a mighty mountain chain stands before him, which 

 must determine the direction of the watercourses and 

 important climatic results. In short, through measure- 

 ment he has achieved perhaps an important addition 

 to our geographical knowledge. As regards modern 

 astronomy, this may almost be defined as the art of 

 measuring very distant objects, and this art has pro- 

 gressed proportionately with the perfection attained in 

 the telescopes and recording instruments employed in its 

 pursuit. 



By the ancients the art of measuring length and volume 

 was tolerably well understood, hence their relatively ex- 

 traordinary advance in architecture and the plastic arts. 

 We hear also of powerful mechanical contrivances which 

 Archimedes employed for hfting and hurling heavy 

 masses ; and the books of Euclid constitute a lasting 

 proof of their power of grappling with the laws regulating 

 the proportion of plane and linear measurement. But 

 with all the mental and mechanical power displayed in 

 those works, it would seem strange that no attempt 

 should have been made on the part of the ancients to 

 utilise those subtle forces in nature, heat and electricity, 

 by which modern civilisation has been distinguished, 

 were it not for their want of the means of measw-ing 

 these forces. 



Hero of Alexandria tells us that the power of steam 

 was known to the Egyptians, and was employed by their 

 priesthood to work such pretended miracles, as that of the 

 spontaneous opening of the doors of the temple, when- 

 ever the burnt offering was accepted by the gods, or as 

 we moderns would put it, whenever the heat generated by 

 combustion was sufficient to produce steam in the hollow 

 body of the altar, and thus force water into buckets whose 

 increasing weight, in descending, caused the gates in 

 question to open. 



Unfortunately for them, the Academia de Cimento of 

 Florence had not yet presented the world with the ther- 

 mometer, nor had Toricelli shown how to measure elastic 

 pressures, or there would at any rate have been a proba- 

 bility of those clear-headed ancients applying the power 

 of steam for preparing and transporting the materials, 

 which they used in the erection of their stupendous 



monuments, and for raising and directing the water used 

 in their elaborate works of irrigation. 



The art of measuring may be divided into the following 

 principal groups. 



First. That of linear measurement, the measurement of 

 area within a plane, and of plane angles ; comprising 

 Geometry, Trigonometry, Surveying, and the constioiction 

 of linear measures, distance meters, sextants and plani- 

 meters, of which a great variety will be found within this 

 building. 



The subject of linear measurement will, I am happy to 

 state be brought before you by one whose name will ever 

 be remembered as the introducer into applied mechanics 

 of the absolute plane, and of accurate measure, 1 mean 

 Sir Joseph Whitworth. It is to be regretted, I consider, 

 that Sir Joseph Whitworth adopted as the unit of measure, 

 the decimalized inch, instead of employing the centimetre, 

 and I hope that he will see reason to adapt his admirable 

 system of gauges, also to metrical measure, which, not- 

 withstanding any objections that could be raised against 

 it on theoretical grounds — that, namely, of not representing 

 accurately the ten millionth part of the distance from one 

 of the earth's poles to its equator — is, nevertheless the only 

 measure that has been thoroughly decimalized, and which 

 establishes a simple relationship between measures of 

 length of area and of capacity. It possesses, moreover, 

 the great practical advantage of having been adopted by 

 nearly all the civilized nations of Europe, and by scientific 

 workers throughout the world. Sir Joseph Whitworth's 

 gauges, based upon the decimalized inch, are calculated 

 to maintain their position for many years, owing to the 

 intrinsic mechanical perfection which they represent, but 

 the boon conferred by their author would be still greater 

 than it is if, by adopting the metre, he would remove the 

 last and only serious impediment in the way of the unifi- 

 cation of linear measurement throughout the world. A 

 discussion will probably arise regarding the relative merits 

 of measurement a bout, of which Sir Joseph Whitworth 

 is the representative, and of measurement a trait, which 

 is the older method, but is still maintained by the Stand- 

 ard Commissioners, both in this country and in France. 



The second group includes the measure of volume or 

 the cubical contents of solids, liquids, and gases, com- 

 prising stereometric methods of measurement, the stand- 

 ard measures for liquids, and the apparatus for measuring 

 liquid and gaseous bodies flowing through pipes, such as 

 gas meters, water meters, spirit meters, of which, likewise 

 a great variety of ancient and modem date will meet 

 your eye, and upon which Mr. Merrifield will address 

 you. 



Another method of measuring matter is by its attraction 

 towards the earth, or, thirdly, the measurement of weight, 

 represented by a great variety of balances of ancient and 

 modern construction. These may be divided into bec.in 

 weighing machines, which appears to be at the same 

 the most ancient and the most accurate, into spring 

 balances and torsion balances. The accuracy obtained 

 in weighing is truly surprising, when we see that a mass 

 of one ten-millionth part of a gramme suffices to turn 

 the scale of a well-constructed chemical balance. Perfect 

 weighing, however, could only be accomplished in a 

 vacuum, and, in accurate weighing, allowance has to be 

 made for the weight of air displaced by the object under 

 consideration. The general result is that the mass of 

 light substances is really greater than their nominal 

 weight implies, and this difference between true and 

 nominal weight must vary sensibly with varying atmo- 

 spheric density. 



Weighing in a denser medium than atmospheric air, 

 namely, in water, leads us fourthly to the measurement 

 of specific gravity which was originated by Archimedes 

 when he determined the composition of King Hiero's 

 crown by weighing it in water and in air. 



Among measures of weight, may be noted a balance, 



