March 30, 1899] 



NA TURE 



519 



OUR ASTRONOMICAL COLUMN. 

 Astronomical Occurrences in April: — 

 April I. I4h. Saturn in conjunction with moon. Saturn 

 2° 16' North. 

 5. Pallas in opposition to the sun. 

 7. i6h. 30m. Transit (ingress) of Jupiter's Sat. III. 

 13. Perihelion passage of Swift's comet (1S99 a). 

 Venus. Illuminated portion of disc = 0750. 



Apparent diameter — 14" '6. 

 Mars. Illuminated portion of disc = o'goo. 



Apparent diameter = 7"'5* 

 iih. 26m. to I2h. 8m. Occullation of |ii Geminorum 



(mag. 3 '2) by the moon. 

 I2h. 35m. Minimum of Algol (3 Persei). 

 loh. 44m. to nh. 8m. Occultation of 3 Cancri 



(mag. 6'0) by the mcon 

 gh. 24m. Minimum of Algol (yS Persei). 

 gh. lom. to loh. 14m. Occultation of /; Leonis 



(mag. 54) by the moon. 

 Epoch of Lyrid meteoric shower (radiant 271° 4- 33°). 

 iih. 12m. to I2h. 20m. Occultation of B. A. C. 4006 



(mag. 57) by the moon, 

 Ceres in opposition to the sun. 



7h. Jupiter in opposition to the sun. At this time 

 the planet will be about 1° from A Virginis (mag. 

 4-6). 

 Polar diameter of Jupiter = 4i"'2. 

 Ceres about 4° N. of i|> Virginis (mag. 5). 

 gh. 53m. to nh. im. Occultation of B.A.C. 5023 

 (mag. 5 '8) by the moon. 



27. Predicted date of perihelion passage of Holmes's 



periodical comet (1892 III.). 



28. llh. 56m. to I3h. 6m. Occultation of fl Ophiuchi 



(mag. 3 '4) by the moon. 

 The planet Jupiter will be well visible during the month, 

 though his position is about 12 degrees south of the equator. 

 The "remarkable hollow in his great southern equatorial belt, 

 and the remains of the famous red spot of 1878-81, may be 

 observed on or very near the central meridian of the planet at 

 the following times : — 



'5- 



25- 



26. 

 26. 



April 7 



17 



April 19 II 23 



24 . ... 10 30 



29 9 38 



S.aturn will be conspicuously displayed in the morning sky, 

 and rises before midnight after the middle of the month. Con- 

 sidered as a telescopic object, however, his low position, nearly 

 22 degrees south of the equator, is a disadvantage, and will seldom 

 allow the details of his surface to appear well defined. 



Orbit of Comet 1S96 III. (Swikt).— Prof. R. G. Aitkeii, 

 of the Lick Observatory, has collected all the observations of this 

 comet that were available, and, after a thorough discussion, has 

 made a definite determination of the orbit {Ast. Nach., Bd. 

 148, Nos. 3550-51). The elements prove to be hyperbolic, and 

 are as follows : — 



T = 1S96, April 17-6473143, C;.M.T. + o-ooo57326d. 



TT = 179° 59' 15-40 + 3 "95 1 



ft = 17S 14 51-48 ± 6-74 \ M. Eq. iSg6-o 



i = 55 34 24-69 ± 8 88 ) 



<] = 0-5662857 + 0-00001347. 



f = 1-0004757 ± 000009985. 

 Saturn's Ninth Sateu.ite. — A few further particulars 

 respecting Prof. W. H. Pickering's important discovery are 

 now to hand. The instrument used was the new photographic 

 doublet, 24 inches aperture and about 160 inches focus, which 

 was presented to the Harvard College Observatory by Miss 

 Catherine Bruce. Attempts have been made in previous years 

 to find satellites by photography, but these turned out un- 

 successful in consequence of the relatively low rapidity of the 

 lens. Last summer, however, the attempt was again made at 

 the Harvard Observatory at Arequipa, Peru, with this new 

 extremely rapid lens. The four successful photographs were 

 taken on the nights of August 16, 17 and 18, 1898, each plate 

 being exposed for about two hours. The number of stars shown 

 on a plate is estimated as 100,000. 



In searching for the satellite two plates were placed film to 

 film, so that each star was indicated by two dots. On the tirst 

 two plates e.\amined an isolated point was found near the planet. 



NO. 1535, VOL. 59] 



A similar isolated point was found on each of the ot^er plates 

 but in different positions with respect to the stars. Ttie plates 

 having been taken at an interval of two days, Saturn had moved 

 in its orbit, and the images on the plates being found tu have 

 moved in the same direction, this furnishes strong evidence of 

 the reality of their being due to a satellite and not to accidental 

 defects of the plates. The new satellite is so faint that there is 

 little possibility of its observation with any but the largest 

 instruments. 



T 



MEASURING EXTREME TEMPERATURES.'^ 

 II. 



Extension of the Range of the Gas-Thermometer. 

 HE methods of measurement so far considered are in a 

 certain sense arbitrary in so far as they depend on extra, 

 polation of empirical formuUi;. If all these methods could be 

 reduced by direct comparison to perfect agreement with each 

 other, a definite scale of temperature would be attained to which all 

 measurements could be referred, and which would leave nothing 

 to be desired from a purely practical point of view. It is prob- 

 able that this scale would not differ much from the theoretical 

 or absolute scale of temperature. For theoretical investi- 

 gations, however, without which no true scientific advance 

 can be made, it is a matter of such fundamental import- 

 ance to refer every measurement to the absolute scale, that no 

 opportunity should be neglected of extending the possible range 

 of accurate observation with the gas-thermometer, because this 

 instrument affords at present the closest approximation to the 

 absolute or theoretical scale. A consideration of the difficulties 

 of the methods of gas-thermometry at present in use will lead 

 naturally to the best methods of extending the range and 

 accuracy of the instrument. 



Defects of Bidb-Methoas. 



In the ordinary method of gas-thermometry a bulb containing 

 the gas is exposed to the temperature to be measured, and the 

 observation consists in determining either the expansion of 

 volume or the increase of pressure of the gas. The principle 

 is very similar to that of the ordinary liquid in glass thermometer, 

 but the apparatus is more cumbrous and difficult to use on 

 account of the necessity of observing both the volume and the 

 pressure of the gas. This method is very accurate at moderate 

 temperatures, but the difficulties increase very rapidly above 

 1000° C. Above 1200° C. it is doubtful whether such measure- 

 ments are of any greater value than those obtained by extra- 

 polation. Apart from the difficulty, which is common to nearly 

 all methods at high temperatures, of maintaining a uniform and 

 steady temperature, the bulb-method of gas thermometry is liable 

 to the following special sources of error. 



(i) Changes in volume of the bulb. 



(2) Leakage and porosity. 



(3) Occlusion or dissociation. 



In order to investigate these sources of error a special form of 

 porcelain air-thermometer (Fig. 3) was designed by the writer, 

 and was constructed in Paris in December 1886, under the super- 

 vision ofW. N. Shaw, F.R.S., of Emmanuel College, Cambridge. 

 A figure and description of this instrument were published in 

 the /"////. Trans. A. ,1887. The same form has since been adopted 

 by MM. Holborn and Wien in their experiments on the 

 measurement of high temperatures at the Reichsanstalt. Thick 

 tubes of 3 sq. mm. cross section, marked AC, BU in Fig. 3, were 

 connected at each end of the cylindrical bulb ba. The length CD 

 could be directly observed at any time with reading microscopes, 

 and the linear expansion of the bulb could be deduced. The 

 volume of the bulb could also be gauged at any time with air, 

 and the mean temperatures of the separate portions ab, ac, 

 BD, could be determined by means of platinum wires extending 

 along the axis of the instrument. This was a most essential 

 part of the apparatus, as the wires afforded a means of accurately 

 reproducing any given set of conditions, and of testing the per- 

 formance of the gas-thermometer at high temperatures in respect 

 of all the various sources of error above mentioned, (i) It was 

 observed that the volume of the bulb underwent continuous 

 changes, chiefly in the direction of contraction, and that the 

 shrinkage was not symmetrical, being apparently greater in the 

 circumference than in the length of the cylinder. (2) To pre- 

 vent leakage, and to close the pores of the material, it is 



I Discourse delivered at the Royal Institution, on March 10, by Prof. H. 

 L. Callendar. F.R.S. (Continued from p. 497.) 



