April lo, 1879] 



NATURE 



531 



F.R.S., gives an exhaustive statement of the experiments 

 that have been made on various targets at Shoeburyness, 

 representing the armour of different ships. A table is 

 given showing the displacement, thickness of armour, and 

 proportion of the former to the latter in ships of different 

 types ; this ratio varies from 6'38 in the Warrior, 4*00 in 

 ^^ Alexandra, to 295 in Xho. Dreadnought, zxidi 2*50 in 

 the Glatton; thus the last may be considered the most 

 heavily-armoured vessel in proportion to size in the navy. 

 The penetration of shot of different diameters and weights 

 with various velocities is given, and the experiments show 

 that it is proportional to the energy of the shot on impact 

 whether due more to velocity or weight, and inversely 

 proportional to diameter of shot ; also that the resistance 

 of solid plates is proportional to the square of their thick- 

 ness. The resistance of composite targets is treated at 

 some length, and a comparison drawn between the 

 various forms adopted in existing ships and the Millwall 

 shield designed by Mr. Hughes, in which the latter is 

 shown to be preferable ; but the questions of steel and 

 steel-faced armour which are now attracting the attention 

 of artillerists are not gone into, and are only referred to 

 with the evident feeling that the end of the battle between 

 guns and armour has not yet come. 



The paper " On the Resistance given to Screw-Ships 

 by the Action of the Screw- Propeller, and how to Remedy 

 it," by Robert Griffiths, points out an important difficulty 

 in screw-propulsion which has only recently been recog- 

 nised. A screw-propeller obtains the resistance to drive 

 the ship forward by accelerating the velocity of the cur- 

 rents of water flowing past the stern of the vessel ; as in 

 different parts of the screw's disk these currents are en- 

 countered at different velocities, the resistance to a blade 

 is not uniform throughout a revolution. In experiments 

 made at Devonport by towing a screw-pinnace, it was 

 found that the water flowed through the lower half of the 

 screw disk nearly at the speed at which the boat was 

 towed, but in the upper half it was so dragged by the 

 boat as to flow past the screw at only half that speed. In 

 dynamometer diagrams, taken with H.M.S. Rattler, it 

 was shown that the thrust of the screw varied from 2*9 

 to 4*1 tons in each revolution. The increase in the resist- 

 ance of the ship, due to the working of the screw above 

 that due to the ship herself when towed at the same speed, 

 and which Mr. Froude has shown to be 40 or 50 per cent., 

 is considerably greater when the upper currents are more 

 accelerated than it would be if the acceleration were uni- 

 formly given to the w hole column of water passed through 

 by the screw disk. The author proposes a screw-propeller 

 so constructed that the blades always meet with equal 

 resistance. The blades are so made that more than half 

 their surface is aft of the centre line, so that the pressure 

 on their surface tends to lessen the pitch ; they are also 

 made movable in the boss, but so connected that by 

 decreasing the pitch of one, that of the other is increased; 

 when, therefore, one blade meets with more resistance 

 than the other, the increased pressure causes it to turn 

 and throw some of the work on the other. 



In his paper on naval gims, Mr. C. W. Merrifield 

 vigorously attacks the Woolwich type of gun, pointing 

 out the disadvantages and absolute futility of the increas- 

 ing twist in rifling at present adopted. It is now four or 

 five years since this was first done by Prof. Osborne 

 Reynolds, and, aided by the Thumierer explosion, it is to 

 be hoped that the time is drawing near when the subject 

 will receive the consideration of the War Department. 

 The author also lays great stress on the advantages of 

 breech-loaders over muzzle-loaders, regarding the latter 

 now, with its complication of gear and fittings, as inferior 

 to the former, even in the simplicity always claimed 

 for it. 



Amongst other papers read at the meeting are the fol- 

 lowing: — "On Sir William Thomson's Navigational 

 Sounding Machine," by P. M. Swan, in which the accu- 



racy of this now well-known apparatus is amply testified 

 by a large number of observations ; and a paper by Mr. 

 J. Scott Russell, F.R.S., "On the true Nature o'f the 

 Wave of Translation, and the Part it plays in Remov- 

 ing the Water out of the Way of a Ship with least 

 Resistance." 



OUR ASTRONOMICAL COLUMN 



Note on 72 Ophiuchi (0.2. 342). —The publication 

 of the entire series of observations of this suspected 

 double star, made at Pulkowa to 1876, does not lessen 

 the difficulty of arriving at a definite conclusion as to its 

 duplicity or otherwise. On November i, 1841, it was 

 noted double magnitudes 4 and 7 on Struve's scale, and, 

 no doubt attached to the observation ; on May 14, 1842, 

 it appeared single, but at the epoch 1842 72 it was again 

 double, the measures giving for position, ii6°'6, and dis- 

 tance, i"*3. Subsequent observations gave the following 

 results : — 



1844-85 



1 845 62 

 1846*49 

 1847-50 



1847-70 



1848-79 

 1850-50 



1851-51 

 1851-67 



1852-63 



Single, or with only a suspicion of elongation at 



63° ; images excellent. 

 With very good images ; no companion seen. 

 Single, or perhaps slightly wedged at 87°. 

 Pos. i62°*4, dist, i"-6i, but there was a doubt if 



the object observed was not an optical illusion. 

 Pos. 168'' -I, dist. I "-6. M. Struve says : " I feel 



sure of the duplicity, but the images are not 



very good." 



Single. 



Pos. i66°'3, dist. i"-49. After the observation a 

 note was added — "This is only an optical 

 deception." 



Single ; under excellent atmospheric conditions. 



This last observation appearing decisive, M. Struve 

 considered that 72 Ophiuchi should be omitted from the 

 list of double-stars, and in the following years only 

 examined it once (i 859-66), when it was again single 

 under very favourable conditions. But in 1876 he found 

 reason to modify his view : at 1876-67 the satellite was 

 seen very distinctly, with position i56°-o, distance i"-6o; 

 a fortnight later there were only very slight impressions 

 of a satellite, and M. Struve remarked that the principal 

 star of 70 Ophiuchi presented an analogous phenomenon, 

 though less distinctly. Hence arose the suspicion that 

 the said impressions were caused by accidental conditions 

 of the air and the instrument. Nevertheless, on consi- 

 dering the preceding observations and the fact of their 

 being made without the least recollection of anterior ones, 

 M. Struve thinks their approximate agreement cannot be 

 attributed to chance, and that we are necessarily led to 

 infer that the star is 'really double, but the companion 

 undergoes considerable and rapid variation of brightness. 

 It is worthy of note that only three weeks before the 

 Pulkowa obser\-ation of 1859, when the star was pro- 

 nounced single, Secchi had recorded of it : " Certainly 

 double, and well separated," his measures giving the 

 position 3°-75, distance o"-6i. 



The Variable Star x Cygni. — According to the 

 later observations of Dr. Julius Schmidt at Athens, it is 

 probable that the next maximum may occur on or about 

 April 25, and the next minimum about December 14. At 

 the last observed maximum on March 14, 1S78, the star 

 was hardly a fifth magnitude, which is about the mean 

 brightness in that phase, the extreme limits of variation 

 being two magnitudes or 4m.— 6m. according to Prof. 

 Schonfeld; at minimum it descends to 13m. No for- 

 mula has yet been deduced which will represent satisfac- 

 torily the totality of the observations, commencing with 

 those of Kirch the discoverer in 1686; considerable 



