September 14, 1893] 



NA TURE 



461 



cates the existence in the observations of a systematic error 

 depending upon the amount by which the measured distance 

 differs from 120°. Reasons for supposing the error to be of 

 subjective origin were indicated. 



A discussion of the data thus corrected furnishes as the value 

 of the constant of aberration 2o"'445±o"'oio 



As subsidiary results of this investigation it appears that the 

 variation in the amount of the refraction from winter to summer 

 is better represented by Regnault's value of the coefficient of 

 expansion of air, 0'003670, than by the values adopted in the 

 tables of Bessel and the Pulkowa Observatory. Also, the obser- 

 vations are in very close agreement with the absolute values of 

 the Pulkowa refractions, but indicate sensible corrections to 

 Bessel's tables. 



Section B (Physics) was prolific of good scientific work. The 

 stereopticon views, with which vice-president Nichols illus- 

 trated his annual address, were a revelation of the astounding 

 resources of photography in depicting phenomena of infinitesi- 

 mal time, the alternating electric current with light and dark 

 intervals clearly depicted, the flight of a bullet and its attendant 

 sound waves shown as if at rest. Prof. Nichols does not think 

 that he-has yet reached the limit of these investigations. Although 

 some of the exposures could only have been for a few millionths 

 of a second, they were always long enough to secure a negative. 



Of equal, if not superior, merit was the delicate and accurate 

 apparatus for measuring expansions, exhibited by Profs. E. W. 

 Morley and Wm. A. Rogers, called the Morley interferential 

 comparator. In a paper read before the section. Prof. Morley 

 explained that he had first described the proposed apparatus be- 

 fore a meeting of the Civil Engineers' Club of Cleveland, and 

 afterwards at the Toronto meeting of this association in 1889. 

 It was first used in a simplified form, in an experiment on the 

 magnetic field, by Profs. Morley and Eddy, which was reported 

 to the association at the Indianapolis meeting in 1890. The 

 present paper was designed to recall to mind the principle of 

 the apparatus and method, as an introduction to a paper by 

 Prof. Rogers, in which several series of experiments with it 

 were detailed, and also as a preparation for an exhibition of one 

 of two forms of the apparatus which have been constructed for 

 use in measuring expansions. These have been constructed by 

 Prof. Rogers, with the aid of a small grant from the research 

 fund of this association. It will measure the expansion of a 

 metallic bar five or ten times as accurately as by old methods, 

 being only limited by the accuracy wilh which temperature can 

 be measured. It consists of two metallic bars, one of steel and 

 one of bronze, with mirrors at each end, so adjusted that any 

 change in adjustment is indicated by interference fringes of 

 sodium light ; 90,000 such fringes to the inch may be readily 

 distinguished and counted. The mirrors are probably the most 

 delicate ever made, being plain within two millionths of an inch, 

 thus far exceeding in accuracy the best objectives of the largest 

 telescopes. 



Prof. Rogers followed with a paper in which he said that pre- 

 liminary to the actual use of the interferential comparator in 

 physical measurements, it was necessary to establish three points 

 with great certainty. 



(l) Does the value of the relative change per degree in the 

 length of steel and bronze bars of metal, expressed in terms of 

 wave lengths, remain constant ? (2) Does the relative length 

 of the two bars compared remain constant at the same tempera- 

 ture after the mirrors have been subjected to extreme tempera- 

 lures ? (3) Does this relative remain constant after the positions 

 of the mirrors have been changed by means of the adjusting 

 screws provided ? 



As a result of many experiments, an aflSrmative answer can be 

 given to the two first inquiries. The change for each degree 

 Centigrade was proved to be 38-31 fringes of sodium light for 

 the steel bar, and 64-23 fringes for the bronze bar of Bailey's 

 metal. When the observed differences in length were reduced 

 to S'l, the point at which the two bars had nearly the same 

 length, it was found that the average probable error in a single 

 comparison was about 072 of a single wave length, including 

 all observations at wide ranges of temperature. 



The answer to the third inquiry was less satisfactory, as oc- 

 casional changes of ten fringes were obtained. The source of 

 this error has, however, been found. In the new vacuo appara- 

 tus, the mirrors have been matched with great exactness. It was 

 then found that the previous matching had been defective. Prof. 

 Morley has computed the maximum effect of this error in chang- 

 ing the apparent relative lengths of the two bars, and has found 

 it to be fifteen fringes. 



NO. 1246, VOL. 48] 



The following are a few of the problems to the solution of 

 which the apparatus has been applied ; — 



(1) The determination of the effect of slow changes in tem- 

 perature upon the relative lengths of the two bars compared. 



(2) The cooling effect of evaporation from a body of water 

 placed near one of the bars. 



(3) Measurement of slow changes in the bars compared due 

 to the near presence of the observer. 



(4) Measurement of the effect of obscure rays of heat stored 

 in large masses of matter in close proximity. 



(5) Measurement of the effect of flexure in changing the length 

 of one of the bars. 



(6) Measurement of changes in length produced by placing 

 one of the bars in a magnetic field. 



(7) Measurement of the heating effect of a current passed 

 through one of the bars. 



(8) Determination of the time required for the complete 

 dissipation of a given amount of heat quickly applied to the 

 bars. 



(9) Proof that air is practically a non-conductor of heat. 



(10) Determination of the value of 100 mikrons in terms of 

 wave lengths of sodium and mercury fringes. . 



Prof. Alexander Macfarlane read a paper on the addition or 

 composition of physical quantities, treating of one uniform 

 method of the addition or composition of scalar quantities at 

 different points, of vector quantities at the same point, of vector 

 quantities at different points, of finite rotations round intersect- 

 ing axes, of finite rotations round non-intersecting axes, and 

 finally of screw motions. The screw motions compounded are 

 not infinitesimal, but may be of any magnitude. 



Profs. Macfarlane and G. W. Pierce contributed a paper on 

 the electric strength of solid, liquid, and gaseous dielectrics, in 

 which it was maintained that for a stratum of air or other gas 

 between two parallel metal plates the electrostatic gradient 

 when the spark passes is less the greater the distance between 

 the plates ; but for paraffined or beeswaxed paper this gradient 

 is constant ; it is also constant for paraffin oil or kerosene. The 

 anomalous behaviour of the gaseous dielectric appears to be due 

 to the greater freedom of motion of the molecules. 



Mr. Joseph O. Thompson lead a paper on "Fatigue in the 

 Elasticity of Stretching." He remarked that attention was 

 first called to the phenomena of elastic fatigue by Lord Kelvin 

 some twenty- eight years ago. He used the elasticity of torsion 

 in his experiments, and demonstrated that in some cases fatigue 

 diminished the slide modulus as much as 6 per cent. Prof. 

 Thompson's paper called attention to the fact hitherto undis- 

 covered that a similar fatigue can be observed in the elasticity 

 of stretching. Its influence in diminishing the Young's 

 modulus amounted in these experiments to less than ^ of i per 

 cent. The wires used were 23m. long, and the metals in which 

 the phenomenon was observed were silver, stet-l, and brass. 



Messrs. F. Bedell, K. B. Miller, and W. F. Wagner con- 

 tributed an elaborate mathematical paper on " Irregularities in 

 Alternate Current Curves." 



At the meeting of Section C (Chemistry) the notable feature 

 was the presentation of Prof. Morley's final determination of 

 the atomic weight of oxygen, giving results obtained by four 

 distinct methods of investigation and with a degree of accuracy 

 that will render this a final determination of this weight, cor- 

 rect to the third decimal figure. Three years ago Prof Morley 

 submitted a preliminary report, in which an account was given 

 of the determination of the ratio of densities of oxygen and 

 hydrogen as 15 '884, correct within one part in four thousand. 

 It has since been found that an accident happened to the appa- 

 ratus during the last experiment of the series, which ought 

 therefore to have been rejected. If this were now to be done, 

 the value would become 15 882. 



Two years ago some account was given of a series of deter- 

 minations of the quantities of water produced from weighed 

 quantities of oxygen and of hydrogen. Twelve experiments 

 were made. In one the quantity of water produced was not 

 determined, owing to accident. From the weights of hydrogen 

 and oxygen consumed, the atomic weight of oxygen was found 

 to be IS8794, with a mean error of ojie part in i6,oco for a 

 single experiment. From the quantities of hydrogen used and 

 of water produced, the value obtained was 15 8792, with a 

 mean error of one part in 7500 for a single determination. 



At the present meeting. Prof. Morley reported the result of 

 twenty determinations of the absolute density of oxygen, and 

 twenty of that of hydrogen. The ratio of these densities found 

 was 1 5 '882. 



