March 23, 1899] 



NA TURE 



497 



come to be regarded as the platinum scale par excellence, and 

 has been adopted as the standard of reference in many recent 

 researches. 



Fusing Point of Platinum. 

 As an illustration of the facility of applying this method, the 

 determination of the fusing point of platinum on the platinum 

 scale may be taken. This is a difficult experiment to perform 

 by any other method. In performing the experiment by the 

 measurement of the electrical resistance, it suffices to take a fine 

 wire of which the electrical constants are accurately known, and 

 to raise it gradually to its melting point by steadily increasing the 

 current. The observation of the resistance of the central portions 

 of the wire at the moment of fusion gives directly the temperature 

 required on the platinum scale. In attempting to perform the 

 same experiment by the expansion method, we are met by the 

 difficulty that the platinum begins to soften and stretch at a 

 temperature considerably below its melting point. Owing to 

 the smallness of the expansion, a very slight viscous extension 

 produces a relatively large error. In the resistance method it 

 is not necessary to subject the wire to tension, and a small strain 

 would in any case produce an inappreciable error on account of 

 the very large increase of resistance with temperature. To 

 obtain an equal degree of accuracy by the calorimetric method 

 (21, or the thermo-electric method (3), it is necessary to use a 

 furnace in which relatively large quantities of platinum can be 

 melted. This has been done by Violle for method (2), and by 

 Barus and Holborn and Wien for method (3). The latter 

 used a linear formula for extrapolation, although their gas- 

 thermometer experiments appeared to indicate a cubic formula 

 for temperatures below 1200° C. 



The temperature of the melting point of platinum on the 

 platinum scale by the resistance method (4) is approximately 

 /i'=i35o°, and varies but slightly for different specimens of 

 platinum. The result when reduced to the scale of the gas- 

 thermometer by assuming that the rate of increase of resistance 

 diminishes uniformly with rise of temperature (according to 

 the usual formula of platinum thermometry, which has been 

 verified with great care at moderate temperatures) gives a 

 temperature of 1820° C. on the scale of the gas-thermometer. 

 It is not improbable that platinum may deviate slightly from 

 this formula at the extreme limit of the scale in the close 

 neighbourhood of its melting point, but the evidence for this 

 result is at least as good as that obtainable by any of the other 

 methods. The observations are very easy and accurate as com- 

 pared with the calorimetric method, and it is not necessary to 

 make any arbitrary assumptions with regard to the formula of 

 reduction, as in the case of the thermo-electric method. 



As the accur.icy of this formula has recently been called in 

 question, on what appears to be insufficient grounds, by certain 

 Ciernian and French observers, it is the more interesting at the 

 present time to show that it leads to a result which cannot be 

 regarded as improbable at the extreme limit of the scale. A 

 different formula has recently been employed Ijy Holborn and 

 Wien, and supported by Dickson {Fliil. Mag., December 1S97). 

 The writer has already given reasons (/"/«/. /l/a,f. , February 1S99) 

 for regarding this formula as inferior to the original, of which, 

 however, it is a very close imitation. The above observations 

 on the melting point of platinum, if reduced by Dickson's 

 formula, would give a result / = i636° C. , which appears to be 

 undoubtedly too low as compared with the results of other 

 methods, however great the margin of uncertainty we are 

 prepared to admit in these difficult and debatable regions of 

 temperature measurement. 



It should be observed that the results of \'iolle by method 

 (2) are consistently lower than those given by the resistance 

 method in the case of silver, gold and copper. We should, 

 therefore, expect a difference in the same direction at the F. P. 

 of Pt. as found by method (4), and not a difference in the oppo- 

 site direction as given by the thermo-electric method, on the 

 arbitrary assumption of a different type of formula for extra- 

 polation at high temperatures. It is a matter of some interest 

 that the assumption of linear f<irmula; for both the specific heat 

 and the rate of change of resistance should lead to results so 

 nearly consistent over so wide a range of temperature in the case 

 of platinum. 



Comparison of tlie Thcnno-couplc and the Plaliniim Tliernw- 



meter, (3) and {a,). 



The chief difficulty and uncertainty encountered by Paschen 



in his experiments on radiation, was that of arranging the 



thermocouple so as to be at the same temperature as the 



NO. 1534, VOL. 59] 



radiating strip of platinum. It is better for this reason to mea- 

 sure the temperature of the strip itself by means of its electrical 

 resistance, the method adopted by Schleiermacher, Bottomley 

 and Petavel. The same difficulty occurs in the direct com- 

 parison of the scales of the thermo-couple and the platinum 

 resistance thermometer. The simplest method of avoiding this 

 objection appears to be that recently adopted by the writer, of 

 enclosing the thermo-couple completely in a thin tube of 

 platinum, which itself forms the resistance thermometer. There 

 can then be no question of diffi^rence of temperature between 

 the two, and the same tube may serve simultaneously for the 

 expansion method, and as a radiating source for bolometric 

 investigation of the law of radiation. The uniformity of 

 temperature throughout the length of the tube can be tested at 

 any time by means of potential leads, or by shifting the thermo- 

 couple to different positions along its length. The method of 

 electric heating is employed, and the central portion only of the 

 tube is utilised in the comparison. 



( To he continued. ) 



THE ORBIT OF THE LEONID METEOR 

 SWARM> 

 T^HE great Leonid swarm of meteors consists of ortho-Leonids 

 which pursue nearly the same path round the sun, and cltno- 

 Leonids which move in orbits sensibly differing from the ortho- 

 orbit. The present investigation is concerned with the ortho- 

 Leonids. They form a dense stream extended along a portion 

 of an immense orbit round which they travel in 33:J years. This 

 orbit has its perihelion a little inside the Earth's orbit, and its 

 aphelion a little outside the orbit of Uranus. It intersects the 

 orbits of these two planets, but lies in a plane inclined to the 

 ecliptic, so that the meteors which traverse it pass under the 

 intervening planets on their outward journey and over them on 

 the homeward journey. 



Accordingly, the orbits of the intervening planets — Mars, 

 Jupiter and Saturn — pass through the orbit of the meteors ; and 

 they, as well as Uranus and the Earth, whose orbits intersect it, 

 and Venus, which lies but little beyond, are well siluated for 

 exercising a perturbating control over the motions of the 

 Leonids. But the inlliience of Mars and Venus is inconspicuous, 

 and that of the Earth only sensible on the meteors which pass 

 close to it ; so that nearly the whole of the perturbating effect 

 upon the greater part of the swarm is due to Jupiter, Saturn and 

 Uranus. 



The procession of ortho-Leonids is so long that it takes 

 between two and three years to pass each point of its orbit : and 

 accordingly when it streams across the earth's path, which it 

 does three times in a century, the earth has time to come round 

 to the point of inter.section in at least two successive years, and 

 on each such occasion receives one of the greater Leonid 

 showers — a splendid spectacle, but of such brief duration, last- 

 ing only a few hours, that it is visible only from the side of the 

 Earth, which happens at the time to be its advancing side. 



The first of these great displays recorded in modern times was 

 that witnessed by Humboldt and Bonpland on the morning ot 

 November 12, 1799, when travelling in South America. It was 

 quite unexpected. .So was the next great shower which visited 

 Europe on the morning of November 13, 1832, and was 

 followed by a still greater display which was seen Ironi number- 

 less stations in America in 1833. This recurrence of the 

 phenomenon after an interval of 33 years led to its being 

 expected in 1S66, and diligent preparations were accordingly 

 then made by astronomers to avail themselves of the oppor- 

 tunity of acquiring more information about the mysterious 

 visitants. These meritorious efforts resulted in a great accession 

 to our knowledge. Prof Hubert A. Newton collected the 

 records of .several ancient observations which showed that the 

 swarm returns to the Earth at intervals of 33I years, and that 

 the date on which the meteors are seen had advanced by 3i 

 weeks since a.d. 902. From their periodic recurrence, he 

 found that they must be moving in one or other of five orbits 

 which he described, and from the advance in the date he 

 inferred that the longitude of the node of the orbit has been 

 advancing, an effect which must be due to perturbations. 

 Prof Adams ascertained which of Newton's five orbits is 



1 " Perturbation of the Leonids." By G. Johnstone Stoney, M..\ , D.Sc, 

 F.R.S., and A. M. W. Downing, M.A., D.Sc , F.R.S. (Abstraot of a 

 paper read before the Royal Society on March 2.) 



