58 



NATURE 



[May 1 8, 1876 



which weighs to the five-millionth part of the body i 

 weighed, sent by Beckers Sons of Rotterdam ; another 

 from Brussels weighing to within a fourteenth millionth 

 part of the weight, in weighing small quantities ; a 

 balance formerly used by Dr. Priestley ; and Professor 

 Hennessy's standards derived from the earth's polar axis, 

 as common to all terrestrial meridians. 



Next comes fifthly, the Measurement of Time, which 

 although of ancient conception has been reduced to 

 mathematical precision only in modern times. This has 

 taken place through the discovery by Galileo, of the pen- 

 dulum, and its application by Huygens to time-pieces in 

 the 17th century. The most interesting exhibits in this 

 branch of measurement are, from an historical point of 

 view, the Italian, German, and English clocks of the 17th 

 century, the Timekeeper which was twice carried out by 

 Captain Cook, first in 1776, and which, after passing 

 through a number of hands, was brought back to this 

 country in 1843, and an ancient striking clock, supposed 

 to have been made in 1348 ; it has the verge escapement 

 which is said to have been in use before the pendulum. 

 The methods employed in modern clocks and watches 

 for compensating for variation of the thermometer 

 and barometer, are illustrated by numerous exhibits, 

 notably the Astronomical Clock, with Sir George Airy's 

 compensation, which will form the subject of a special de- 

 monstration by Messrs. Dent and Co. 



The measurement of small increments of time has been 

 rendered possible only in our own days by the introduction 

 of the conical pendulum, and other apparatus of uniform 

 rotation, which alone conveys to our minds the true con- 

 ception of the continuity of time. Among the exhibits 

 belonging to this class, must be mentioned Sir Charles 

 Wheatstone's rotating mirror, moved by a constant falling 

 weight, by which he made his early determination of the 

 velocity of electricity through metallic conductors ; the 

 rotative cylindrical mirror, marked by successive electrical 

 discharges, which was employed by Dr. Werner Siemens 

 in 1846, to measure the velocity of projectiles, and has 

 been lately applied by him for the measurement of the 

 velocity of the electric current itself, and the Chronometric 

 Governor, introduced by him in conjunction with myself, 

 for regulating Chronographs, as also the velocity of steam 

 engines under their varying loads ; Foucault's Governor, 

 and a considerable variety involving similar principles of 

 action. 



Another entity which presents itself for measurement 

 is, sixthly, that of Velocity, or distance traversed in a unit of 

 time, which may either be uniform or one influenced by a 

 continuance of the cause of motion, resulting in accelera- 

 tion, subject to laws and measurements applicable both in 

 relation to celestial and terrestrial bodies. I may here 

 mention the instruments latterly devised for measuring 

 the acceleration of a cannon-ball before and after 

 leaving the mouth of the gun, of which an early example 

 has been placed within these galleries. Other measurers 

 of velocity are to be found here. Ships' Logs, Current 

 Meters, and Anemometers. 



In combining the ideas of weight or pressure with space, 

 we arrive at seventhly, the conception of work, the unit of 

 which is the foot-pound or kilogrammetre, and which, when 

 combined with time, leads us to the further conception 

 of the performance of duty, the horse-power as defined 

 by Watt. The machines for the measurement of work, 

 here exhibited, are not numerous, but are interesting. 

 Among these may be mentioned Professor CoUadon's 

 Dynamometrical Apparatus constructed in 1844 ; Richard's 

 Patent Steam Engine Indicator, an improvement on Watt's, 

 and Mr. G. A. Hirn's Flexion and Torsion Pandynamo- 

 meters. 



Eighth. The Measurement of Electrical Units — of elec- 

 trical capacityof potential — and Resistance, forms a subject 

 of vast research, and of practical importance, such as few 

 men are capable of doing justice to. It may be questioned, 



indeed, whether Electrical Measurement belongs to the 

 province of mechanical science, involving, as it does, 

 problems in physical science of the highest order ; but it 

 may be contended on the other hand that at least one 

 branch of Applied Science, that of Telegraphy, could not 

 be carried on without its aid. I am happy to say that 

 this branch of the general subject will be brought before 

 you by my esteemed friend Sir WiUiam Thomson, than 

 whom there is no one more eminently qualified to deal 

 with it. I may, therefore, pass on to the next great branch 

 of our general subject, the ninth : Thermal Measurement. — 

 The principal instrument here employed is the thermometer, 

 based in its construction, either upon the difference of ex- 

 pansion between two solids, or on the expansion of fluids 

 such as mercury or alcohol — (the common thermometer) or 

 upon gaseous expansion (the air thermometer) ; or again, it 

 may be based upon certain changes of electrical resist- 

 ance, which solids and liquids experience when subjected 

 to various intensities of heat. With reference to these, the 

 air thermometer represents most completely the molecular 

 action of matter which is the equivalent of the expansibility. 

 I shall not speak of the different scales that have been 

 adopted by Rdaumur, Celsuis and Fahrenheit, which are 

 based upon no natural laws or zero points in nature, and 

 which are therefore equally objectionable upon theoretical 

 grounds. Would it not be possible to substitute for these 

 a natural thermometric scale ? One commencing from 

 the absolute zero, of the possible existence of which we 

 have many irrefutable proofs, although we may never be 

 able to reach it by actual experiment. A scale com- 

 mencing in numeration from this hypothetical point 

 would possess the advantage of being in unison through- 

 out with the physical effects due to the nominal degree, 

 and would aid us in appreciating correctly the relative 

 dynamical value of any two degrees of heat which could 

 be named. Such a scale would also fall in with the 

 readings of an Electrical Resistance Thermometer or 

 Pyrometer, of which a specimen has been added to this 

 collection by myself. 



When temperature or intensity of heat is coupled with 

 mass we obtain the conception of quantity of heat, and if 

 this again is referred to a standard material, usually water, 

 the unit weight of each being taken, we obtain what is 

 known as specific heat. The standard to which measure- 

 ments of quantity of heat are usually referred is the heat 

 required to raise a pound of water one degree Fahrenheit, 

 or the cubic centimetre of water one degree Centigrade. 



The most interesting exhibits in this branch of measure- 

 ment, are, from an historical point of view, the original spirit 

 thermometer of the Florentine Academiadel Cimento, and 

 the photographs of old thermometers ; the original La- 

 voisier Calorimeter for measuring the heat disengaged in 

 combustion, Wedgwood's and Daniell's Pyrometers. 



As illustrating modern improvement may be instanced 

 a long brass-cased thermometer showing the variation in 

 the readings, when the bulb and when the whole ther- 

 mometer is immersed ; a thermometer with flat bulb to 

 improve sensitiveness ; a thermo-electric alarum, for 

 giving notice when a given temperature is reached ; an 

 instrument for measuring the temperature of fusion by 

 means of electric contact invented by Prof Himly ; 

 Dr. Andrews' apparatus for measuring the quantity of heat 

 disengaged in combustion ; Dr. Guthrie's diacalorimeter 

 for measuring the conductivity of liquids for heat, and a 

 thermometrictu be by Prof Wartmann for determining the 

 calorific capacities of different liquids by the process of 

 cooling. 



Finally, Joule has taught us how to measure the unit of 

 heat dynamically, and the interesting apparatus employed 

 by him from time to time in the various stages of the 

 determination of this most important constant in applied 

 mechanics, are to be found, rightly placed, not among 

 thermometers, and other instruments placed in the 

 physical sections, but among the instruments required in 



