October 26, igi 1] 



NATURE 



53 



<=> 



is a variation of Very long period or a secular change we 

 must in the next few years experience a preponderance of 

 summers with more than the average number of " hot " 

 days. E. Gold. 



ad Garden Suburb. N.W., October 23. 



Determination of Refractive Index of a Liquid. 



The following simple method of finding the refractive 

 index of a liquid available in small quantities may be of 

 interest. 



A plane mirror A is placed on the base of the stand, and 

 on it is put the double convex lens in such a position that 

 its centre is beneath the needle point B. With the eye 

 directly above B, the observer adjusts the sliding arm until 

 the needle point and its image just coincide, as found bv 

 parallax. The distance from B to the centre of the lens 

 is then accurately found — let it be /,. 

 The experiment is then repeated, after 

 fust placing a drop of the liquid upon 

 the mirror, when it will be spread out 

 to a plano-convex lens between the 

 glass lens and the mirror — let the new- 

 focal distance be f„ ; then evidently the 

 focal length / of the liquid lens will be 

 given by 1//= i// 2 — i//,. 



But since the focal length of the 

 liquid lens is also given by the relation 

 i/f=(fi—i)i'r, where r is the radius of 

 curvature of. the surface of the glass 

 lens, it is evident that from a know- 

 ledge of r the index of refraction of 

 the liquid can be at once found. 



If r is not known it can be found by 

 putting a sheet of paper between the 

 lens and mirror, and again obtaining 

 an image of B coincident with itself by 

 reflection in the lower surface of the 

 lens. If this new distance from the 

 tens be called d, we have, since reflection is now onlv at 

 the upper surface of the lens, /i/r—i/d=(fi—i)l—r, or 

 f = (2/i— i)d, where y. now, of course, refers to the glass, 

 and can, if necessary, be calculated. 



The apparatus is thus complete in itself, and three read- 

 ings of the position of B give all the data required. 



G. N. PlNGRlFF. 

 Market Bosworth Grammar School. 



The Nematodes of the Thames. 



Is a recent letter to Nature on the " Ooze of the 

 [Thames," I alluded to the number of nematodes which 

 1 had observed. I found as I continued my researches at 

 li ast three different species were present. I have since 

 been working on some ooze from near the Tower Bridge, 

 and again find three different species, some of which are 

 quite distinct from the forms taken at Kew. Thus the 

 two localities yield at least four, if not five, different kinds. 

 They range from about 3 mm. to 20 mm. or more in 

 length. Considering the important part which some of 

 these lowly creatures play in human and animal pathology, 

 it would seem that the Thames mud offers a wide field 

 for investigation. May we hope that this note will direct 

 the attention of London naturalists to a subject of great 

 importance lying close to hand? 



Swadlincote. Hilderic Frieniv 



Miniature Rainbows. 



With reference to the recent correspondence on minia- 

 ture rainbows, there is, or was, a most perfect example 

 at the beautiful waterfall known as "Stock Gill Force," 

 situate half a mile outside the little town of Ambleside, 

 near the head of Lake Windermere, County Westmor- 

 land. 



About five o'clock in the evening is the best time to see 

 it, and, of course, the sun must be shining. 



Richd. Coulson. 



4 Waltham Terrace, Blackrock, Co. Dublin, 

 October 15. 



NO. 2igi, VOL. 8/] 



Olive Trees. 



In this part of the world when an olive plantation is 

 being made trees of more than inches in diameter in 

 the stem are put in, with all roots cut off short. In the 

 hole where they are planted about two handfuls of barley 

 are also put in. 



Considering the age of the trees and the way the roots 

 are cut away, it would seem impossible that the tree would 

 ever grow. 



These facts may be of interest ; and I should like to 

 know what useful purpose is served by the barley. 



Smyrna, Asia Minor. Dora Barfield. 



EXPLOSIVES ON BOARD BATTLESHIPS. 



SOME people have been tempted of late to look 

 back to the old days when black powder held its 

 sway, indifferent to the effects of temperature, and 

 always to be trusted so long- as it was kept dry. 

 "Villainous saltpetre" it was called, with a rough 

 affection, and after storing it on shore in magazines 

 plumed with lightning conductors, or comfortably near 

 the boilers on board ship, we never gave it a thought 

 until it was fed, in its flannel bag, into the gun. 

 Then came armour and the long and hard contested 

 duel between protection and penetration. The velocity 

 of the projectile had to be increased. The old black 

 powder, treat it as we would, could only deliver its 

 rather clumsy blow which, while it imparted but a 

 low velocity to the shell, gave an unpleasant, percus- 

 sive strain to the gun. It was a push that these 

 heavier projectiles required, not a blow. 



This state of things led to the introduction of slow- 

 burning powders, and they progressed very slowly. 

 First, the size of grain of black powder was increased 

 until it attained the dimensions of a two-inch cube. 

 The improvement, however, was slight, and the in- 

 ternal stresses on the bore of the gun were still too 

 great. The next step towards real progress was the 

 introduction of what was known as cocoa, or brown 

 powder, in which materials other than dogwood or 

 alder furnished the carbon. This had but a short 

 reign, its place being taken by various propellants 

 which were franklv chemical compounds. To give a 

 list of all this class of propellant, with which experi- 

 ments and lens-thv trials have been carried out with 

 more or less success, would be far beyond the scope 

 of this article; but the interest of the scientific world 

 gradually focussed itself on compounds having nitro- 

 cellulose as their chief constituent, especially after the 

 wonderful discovery that two high explosives, gun- 

 cotton and nitroglycerine, when chemically com- 

 pounded, tamed and" restrained each other, so that a 

 propellant resulted of which the speed of burning 

 could be regulated at will by increasing or diminishing 

 the size of the grain or cord, and also by adjusting 

 the proportions of its constituents. Thus came to 

 man's hand a propellant which far outstripped black 

 powder even in its improved forms, and threw into 

 shade the cocoa powder which, after all, remained in 

 the category <-n which the French artillerists con- 

 temptuously alluded as poudre brutalc. 



" Now," ' triumphantly exclaimed the gunmakers, 

 "here is a propellant which will enable us to increase 

 the striking velocity of our armour-piercing projec- 

 tiles, and also to flatten our trajectories to an extent 

 never before contemplated. The cemented steel plates 

 will no longer confer invulnerability on the battle- 

 ship, and our chances of hitting her will be vastly 

 enhanced." So, to suit the new- powders, guns were 

 lengthened and charges increased, and the muzzle 

 velocities rose by leaps and bounds, while the destruc- 

 tive stresses ori chamber and bore were minimised 

 bv the employment of a propellant the characteristics 



