KNowij.nr.i:. 



Jink, 1012. 



prepare tlic iliagram sliewii in Figure J4f). Tliis 

 consists of two squares drawn close together and 

 whose sides are sufficiently long to take the scales of 

 declination and K.A. In these squares are con- 

 structed the two views which go to form the 

 stereoscopic slide. Now we have to consider the 

 matter in three dimensions and the squares represent 

 the two dimensions in the |)lane of the jjajjer. The 

 third dimension, at right angles to the plane of the 

 pajier, must now he represented. To do this select 

 a |)oint about the middle of the left-hand scpiare, 

 which will he tlie vanishing point of all the lines in 

 that square running at right angles to the plane of 

 the paper. To fi.\ the corresponding point in the 

 other square draw a horizontal line through this 

 point and measure along it towards the right-hand 

 figure a distance equal to the distance between the 

 two eyes (about three inches) ; this will be the 

 recpiired point. These points are lettered L and R 

 in Figure 246 because L is the vanishing point for 

 the view obtained by the left eye and R that 

 for the right eye. The four corners of the square 

 should now lie joined to the corresponding 

 vanishing point. It will be well to examine 

 the drawing in a stereoscope at this stage to 

 make sure that all is right. The view obtained 

 should have the appearance of looking down a ver\- 

 long square tunnel. None of the lines should 

 appear double. We will now consider the con- 

 struction about the left-hand figure only, it being 

 understood that a similar construction on the right- 

 hand figure is necessary. Along the line BL a scale 

 must be made to indicate depth in the figure. In 

 our case this must correspond to stellar magtiitudi-. 

 It is wise to make this scale short compared with 

 the length BL. The graduations on this scale must 

 in each diagram be proportional to the cosine of the 

 angle corresponding to LBC. A simple method of 

 securing this is to drop a jierpendicular from C on 

 to the line BL and take a portion of the distance 

 from B to the foot of the |)erpendicular. and divide 

 it into a suitable tiimihir of eipial p;irts. In 

 l'"igure 246 one-third of tliis distaiuc was taken 



and divided into eight equal parts. The point B 

 then represented the graduation for magnitude 1 -8, 

 and each subse(|uent graduation indicated an increase 

 of -2 in magnitude. It must be clearly understood 

 that this scale in the left-hand figure will be different 

 from that in the right-hand figure : each scale must 

 be constructed independentlx'. 



The position of each star in I'^igure 246 has been 

 fixed b\' the same method, but in order to make the 

 matter more clear, the construction for fixing the 

 [position of one star onlv (viz., o Ursae Majoris) is 

 shewn. For convenience of reference we will call 

 the line BC (along which the scale of R.A. is made) 

 the axis of .v, and the line B.-\ (along which the 

 scale of declination is made) the axis of y. The 

 line BL, on which we have our magnitude scale, we 

 w ill call the axis of z. First fix the position of the 

 star, as it appears in Figure 245, by drawing a line 

 from declination = 57" 32' parallel to the axis of 

 .V. until it meets the line drawn from R.A. = 12'' 10'" 

 [)arallel to the axis of y. Call this point o. F"rom 

 tile point on the axis of z. corresponding to magnitude 

 .5-4. draw a line parallel to the axis of x until it 

 meets the line running from point R.A. = 12'' 10'" to 

 L. From this point of intersection draw a line 

 parallel to the axis of y, until it meets the line running 

 from () to L. This last point is the position of the 

 star in the diagram. It will be remembered that we 

 took the point B, on the axis of z, as representing 

 magnitude 1-8, which is that of e L'rsae Majoris. 

 the brightest star in the constellation. The fixing of 

 the position of this star in the diagram is particularly 

 easy, as it requires no reference to the axis 

 of z at all. A very little practice is sufficient to 

 enable one to make these slides with great ease and 

 rapidit}-. 



It is a great advantage if the construction is done 

 on drawing paper, and the final positions of the stars 

 pricked through on to the final slide, which then 

 shews none of the construction lines. The slides 

 present a much more attractive appearance if they 

 are of black paper, and the position of the stars 

 inaikeii by spots of Chinese white. 



NOIKKS. 



THE ARGENTINE METEOROLOGICAL OEFICE.— 

 In an article which the Director, Mr. Walter G. Davis, has 

 contributed to Symons's Meteorological Magazine for 

 April, we learn that the Meteorological Service in the 

 Argentine Republic was established in the year 1872. At the 

 present time it consists of thirty-five stations of the first order, 

 equipped with self-registering instruments; one hundred and 

 fifty-six of the second order, whore observations are made at 

 8 a.m., 2 p.m., and (S p.m.; ten of the third order similar to the 

 second but without barometer, and one thousand six hundred 

 rain gauge stations. 



THE TITANOTHERES.— The front page of The 

 Scientific American for .\pril 6th, 1912, is occupied by a 

 photograph of the restored head of the largest of the 

 Titanotheres — giant-horned monsters allied to the Rhino- 

 ceroses. Some of the animals averaged eight feet high at 



the shoulder, and sixteen feet in length. For comparison 

 there is also shown in the picture a restored head of an 

 ancestral member of the family, (luite hornless, and about 

 the size of the smallest Shetland pony. The lieads form part 

 of an exhibit which is being installed in the hall of \'ertebrate 

 Palaeontology, in the Museum of Natural History, New 'S'ork. 



BEHAVIOUR OF METALLIC ALLOYS WHEN 

 H1:aTED in a vacuum.— r/(f Cliemical Xetis gives a 

 suunnary of experiments by Messrs. Clarence Richard Groves 

 and Thomas Turner, from which it appears that two or more 

 metals may volatilise together. Thus lead and iinc tend to 

 pass over together. In the iron-zinc series also there is an 

 increasing proportion of the iron carried over as the 

 temperature rises from 500'. In the silver-zinc series, 

 although separation is nearly ciuantitative at 700'~, there is an 

 increased loss of silver with higher temperatures. 



