148 



NATURE- 



{Dec, 19, 1878 



nose gently against his arm, after whicTi he walked off with a 

 great sigh of relief. 



A story somewliat analogous to the foregoing was told me by 

 a friend, whose uncle, an old country squire in one of our 

 western counties, had a favourite hunter in a loose box in the 

 stable. One warm summer day he was "athirst," 'and could 

 get no water. He tried to draw the groom's attention to 

 Sie fact, but without success. The horse was not to be dis- 

 couraged ; he evidently gave the matter consideration. The 

 thirst was pressing. All at once he remembered that he always 

 had a certain halter put upon his head when led to the water. 

 He knew where it hung. He managed to unhook it from its 

 peg, and carried it to the groom ! who at once, in great admira- 

 tion of the knowledgable brute, rewarded him in the manner he 

 desired. M. Carey-Hobson 



Colour-Blindness 



Dr. Pkior's letter is almost entirely philological, and there- 

 fore does not come within my province. I have alluded to the 

 coloar-blLnd impression of white in my paper in the Phil. Trans. 



I should like to know more about the eyesight of the person 

 who says he cannot distinguish snow. 



The latter part of the first paragraph of my letter on p. 1 20 

 should run, " In pigments, neutral green appears to me gray. I' 



December 14 W. TOLE 



Magnetic Storm, May 14, 1878 



Referring to a letter from the Rev. S. J. PeiTy in Nature, 

 vol. xviii. p. 617, reporting the magnetic disturbances observed 

 at Stonyhurst, Melbourne, and Shanghai, on May 14, it may 

 interest your readers to learn that earth-current disturbances 

 were also noted on the Persian Gulf cables from 4 p.m. (Kurra- 

 chee time) on the 15th up to 5 a.m. on the following day. 



Unusually strong earth- currents were also observed on June 3 

 and 4, on the cables between Bushire and Kurrachee ; the 

 current-strength at 2.40 P.M. on the 3rd, and 12.20 A.M. on the 

 4th, being reported as equal to fourteen Daniell's cells. 



Kurrachee, November 8 Henry C. Mance 



" Measuring the Height of Clouds " 



»T. The electric light promising to be of great intensity at a 

 small cost, tlie thought occurred to me that it might be used with 

 advantage for the purpose of ascertaining the height of clouds. 

 For, supposing an electric lamp sending a beam of light to the 

 clouds, the spot where the light meets the latter, will be more or 

 less visible, and we are obviously able to determine trigono- 

 metrically the height of the cloud. 



By using two lamps, or a lamp and two reflectors, we may 

 easily find also the rate at which clouds travel, by bringing the 

 plane, passing through the axes of the beams of light, parallel to 

 the direction in which the clouds move, and by noting the time 

 it takes a cloud to travel from one beam of light to the other, 

 having, of course, determined also the actual distance between 

 the two spots of light on the clouds. 



■ The above refers to observations during the night only, but by 

 making use of coloured light, or by bringing a substance in the 

 carbon? of the lamp, the spectrum of which is easily recognisable, 

 ■we might probably be able to M'ork also during day-time. 



Kew J. F. WiLKE 



The Weather 



After a week of unusually cold weather, the mean tempera- 

 ture having been 2S°'5, and the wind constant from a northerly 

 point, a thaw set in yesterday, and the ^^'ind became westerly, 

 when immediately after sunset a rather unusual condition of 

 weather occurred : viz. , the rapid formation of a complete 

 sheet of ice on the roads, though at the time, and till eleven 

 I'.M., the thermometer was 2" or 3° above the freezing-point. 



As the sky was overcast at the time radiation cannot well 

 account for it. Owing to the penetration of the cold, the 

 surface must have retained a temperature considerably below 32° 

 for some time after the air had become warmer and damper, so 

 that the moisture was at once congealed. . > . 



Clifton, December 16 



G. S. Thomson 



THE LAST EXPERIMENTS WITH THE 



'. ■ ■'■ So-TON GUN •;•'/' 

 n"* HE last experiments with the 80-ton^ giin at Wool- 

 -^ vvich deserve to be recorded, if only for the sake of 

 showing that our scientific artillerists appear to be working 

 in the proper channel. The last shot fired from the 

 monster piece of ordnance was with the unprecedented 

 charge of 460 lbs. of powder, and yet there was not so 

 much strain upon the gun as that fonnerly exerted by 

 charges one hundred lbs. less. The reason of this is 

 in the main due to a change having been made in the 

 character of the gunpowder employed ; for whenever 

 the former powder was used, even in lesser quantity, the 

 pressure of the gas inside the gun rose at once. This 

 would not so much matter if it could be 'shown that with 

 the increase of strain, the work of the shot increased 

 also. But such is not the case. For instance, in the 

 case of two shots fired last week, one was sent on its 

 way by 460 lbs. of prismatic powder, recording a velocity, 

 we are told, of 1,626 per second, and a strain inside the 

 gun of 19I tons, while the other, with but 425 lbs. of cube 

 powder, had a speed of only 1,600 feet, while it exerted 

 a strain upon the weapon of 21 tons per square inch. 

 The gun has been chambered — or in other words the 

 cartridge cavity enlarged — to permit the introduction of 

 heavier charges, as also to allow of a certain amount of 

 air-space in the cartridge ; but this modification in the 

 weapon, beneficial as it may be, does not account, as we 

 have shown, for the decrease upon the strain of the gun. 

 This is due to the change in the powder. 



In most of the former experiments a gunpowder of solid 

 cubes, irregular in shape and measuring about an inch 

 and a half, were employed ; the recent results have been 

 secured by thick six-sided prisms, about an inch across, 

 and so accurately shaped that they may be packed to- 

 gether very closely. There is a single perforation in the 

 middle of this prismatic powder, which, by the way, is of 

 Gemian origin, and when the cartridge has been securely 

 packed so as to represent one solid mass, the perforations 

 running through the whole length of the charge permit of 

 the same being rapidly kindled. If the perforations were 

 not there, half the charge would probably be expelled the 

 gun before it was kindled ; so that a packed cartridge of 

 prismatic powder represents as nearly as possible a solid 

 charge with tubes running its entire length, through which 

 the kindling flames pass. 



It has, of late, grown to be an axiom that the larger 

 the gun the larger must be the grains of powder. A large 

 grain of gunpowder burns slow because the fire is some 

 time reaching the centre, and a slow-burning powder is ' 

 what artillerists require for rifled guns. In a smooth- 

 bore weapon the cannon ball fits loosely, and may be 

 expelled at a bound ; but in rifled cannon the shot, so to 

 speak, moves upon a sort of railway, and it would never 

 do to get the shot into motion too suddenly. An undue 

 strain would be exerted upon the gun, while the velocity 

 of the shot would not be increased. For a rifled gun, 

 therefore, a slow- burning charge is absolutely necessary, 

 and this is to be secured only by reducing the surface to 1 

 be kindled. In the case of the prismatic powder, the ' 

 grains, if they may be called by that name, are so closely j 

 packed that no fire can get between them, and hence the 

 action of kindling is still further reduced. j 



Not only is the shape and density of powder grains i 

 now attracting particular attention, but the percentage j 

 of moisture contained in the material has also lately been j 

 under study. The amount of water in gunpowder to the \ 

 minute extent existing in ordinary samples is found to in- j 

 fluence combustion in a very marked degree, and nothing ; 

 but an exhaustive series of trials can give sufficient data ^ 

 for practical application of so important an element in | 

 . the science of explosives. In the meantime chemists are j 

 pointing out yet another source of uncertainty in the ; 

 combustion of gunpowder, to which, aotwithstsndiog i 



