330 



KNOWLEDGE & SCIENTIFIC NEWS. 



[January, igo6. 



Answers to Correspondents. 



Ignoramus. Altitude of the Sun. Many books on Practical 

 .Astronomy define the terms used, though not in a very explicit 

 manner. Perhaps the following may make it clear to you :— 

 The altitude of the sun is the angle at the observer's eye between 

 the sun and the horizon. The dultnatwn of the sun is the angle 

 between it and the Equator (produced). The declination at 

 noon each dav is given in any astronomical ephemeris ^i'i./<■ 

 " Science Ve.ar Book "). Therefore, knowing the declination, 

 one can find the altitude at any given latitude by subtracting 

 the latitude from go', and uddint; the declination if summer, or 

 suhlnicliii^ it if winter (when sun is south of Equator). Thus, 

 if latitude 51° 28', 



Deducted from 90° = 38"' 32' ; 

 Declination at noon January 1st = 28= 3'28" ; 



.-. altitude - 10° 28' 32". 

 The following diagrams explain this, where A is the position 



of the observer. Since the sun is at a comparatively infinite 

 distance, the altitude will be practically the same at O (centre 

 of the earth) as at A. Tben in fig. 2, \ O E is a right angle, as 

 is A O II. Deducting the common angle A O E (= the 



e^-y—i 



latitude), we have N O A (= 90^ - latitude) = E O H, and 

 E O H + Declination is the altitude. This, of course, is the 

 principle for finding one's latitude at sea. Observe (with a 

 sextant) the altitude of the sun above the horizon, add or de- 

 duct the declination las given in tables), and deduct the result 

 from 90-. This gives the latitude. 



Cecil Evans. Though we have referred your letter to several 

 authorities, we fear it is not sufficiently explanatory for us to 

 comprehend your meaning. 



G. P. Blake. Remember that the obliquity is not constant. 

 It is diminishing nbotit o"4 annually, though the e.xtiet diminu- 

 tion can only be surmised by theo'ry. The change should not 

 be " put down to precession." 



To Ma.ke Iron Grow. 



The Franklin Institute has awarded the Elliot Cresson Gold 

 Medal for a new process by which iron can be made to grow. 

 This consists in heating and cooling the bar of iron which it is 

 intended to magnify to a " critical " temperature a number of 

 times. The results are extraordinary. To the Mechanical 

 Science Section of the American .Association two bars of iron 

 cast in one mould were presented for critical inspection. One 

 bar remained exactly as cast. The companion bar had been 

 caused to grow gradually in cubical dimensions till it is now 

 46 per cent, larger than the other, the weight remaining the 

 same as before expansion. Both bars were machined on one 

 side to show the texture and metallic appearance ; and it was 

 difficult to detect any change except the very apparent difter- 

 ence in size. It is said that important practical applications 

 ha\e already been found for this remarkable discoverw 



ASTR.ONOMICAL. 



By Charles P. Bltler, A.R.C.Sc. (Lond.), F.R.P.S. 

 Diffraction Grating R_eplicas. 



I'~oK some years past both physicists and astronomers have 

 been using the excellent Diffraction Grating Replicas made by 

 Mr. T. Thorp, of Manchester, and now he is able to make the 

 interesting announcement that he has been successful in pro- 

 ducing coiteave replicas. There will be a very great field for the 

 application of this form of spectroscope, which is self-contained 

 and needs neither coUimating nor telescope lenses. At pre- 

 sent the replicas are only of plain celluloid, but experiments 

 are in progress for silvering the diffracting surface, when the 

 brightness of the spectra will be greatly enhanced. 



.\nother distinct advance is the making of the grating films 

 on glass rings, so that there is no glass for the light to pass 

 through, and these will be very useful for investigations of 

 ultra violet radiations. 



Possible Relationship between Light and 

 Velocity Variations of Variable Stars. 



In a recent paper by K. C. Curtiss the author points out an 

 important coincidence which he has found to exist between 

 the variations of magnitude and radial \elocity of the variable 

 star W Sagittarii. When these were plotted it was at once 

 seen that there was a most striking concordance, the two 

 curves corresponding not only in general form, but in most of 

 the minor inflections. The light maximum, according to the 

 latest values, occurs o'gg day after periastron, and almost 

 exactly at the epoch of velocity minimum. Comparing the 

 most recent photometric measures of the light variation, from 

 the Harvard College Observatory, with those made by 

 Schmidt some thirty years previously, there are seen to be 

 numerous differences, and it seems not improbable th.at these 

 may be more or less due to actual changes in the system of 

 this star during the interval which has elapsed between the 

 two series of observations. 



New Views about Comets. 



Various novel facts concerning the formation and appear- 

 ance of the tails of comets have recently been described by 

 Professor Barnard, the discussion of which leads him to think 

 that the views generally accepted may have to be consider- 

 ably modified in future. 



Although no explanation has been definitely found to cover 

 all the many details exhibited bv the growth and dissipation 

 of a comet's fail, that of Bredechin has probably received 

 most favour. This ascribes the varying forms of tail to dif- 

 ferent constitution of the matter evolved from the comet, on 

 which the resulting effects of the sun's repulsive force will be 

 to produce streams of cometary matter extending in curves of 

 varying degree, depending on the density or other physical 

 properties of the substance. In this and most other theories 

 it has been customary, however, to ascribe practically all the 

 cause of the phenomena to the action of the sun, supposing 

 the comet to merely supply the material to be acted upon. 



During the last ten or twelve years, however, the applica- 

 tion of photography has resulted in the discovery of many 

 new features which are rather of a revolutionary nature, and 

 are not easily explained. Professor Barnard now thinks that 

 three main causes are at work. 



First, the sun, which produces in the nucleus of the comet a 

 disturbing action, and which influences the general direction 

 of the tail-producing particles. Second, the comet itself, in 

 which a strong ejective power seems to be located. This is 

 proved by the frequent formation of straight minor tails or 

 streams of particles which are often seen to issue at large 



