58 



NATURE 



[May 19, 1887 



temperature of about 43°" 5 over the whole area, and con- 

 firm all the provisional conclusions stated above. The 

 figures observed in the three typical positions are as 



Place ... Channel. Off Skate Island. Strachur. 



Date ... March 30. March 28. March 29. 



Temp, surface 

 bottom 



447 

 44 '2 



43-8 

 43 "9 



44 '9 

 45-5 



From the forms of the curves the spring minimum ap- 

 pears to be past, and over all the temperature is about 

 2° -5 higher than at the same period last year. The water 

 in Upper Loch Fyne is now cooling at the bottom and 

 heating again on the surface, the range for the year at 

 great depths having been only 4°, The actual change of 

 temperature in the sea-area between the beginnmg] of 

 February and the end of March is very slight, but it is 

 significant in showing by its direction that the period of 

 minimum lay between the two. 



One interesting application of the observations may be 

 made to climatology. A great proportion of the heat 

 gained is derived not from solar radiation, or the contact 

 of heated air with the surface, but from the warm Atlantic 

 water entering by the tides. Since the water on the 

 Plateau appears to remain warmer than that inside all the 

 year round, no heat is lost to the Atlantic in winter ; but 

 all must be radiated off from the surface or employed in 

 evaporating water or heating air by contact, and in this 

 way more heat is returned to the air of the Clyde sea-area 

 in winter than was received from it in summer. Another 

 observation may be mentioned which serves to show how 

 important a bearing temperature observations may have 

 on biology. On February 4, four tow-nets were used 

 off Strachur at different depths : one at 70 fathoms, one 

 at 50, one at 30 fathoms deep, and the fourth at the sur- 

 face. There was nothing in the surface-net, and the 

 surface temperature was 43°. The contents of the other 

 three nets were examined by Mr. David Robertson, of 

 Millport, who reports : — " In all three nets Copepoda were 

 moderately abundant. The nets at 70 and 30 fathoms 

 contained one and the same species ; but the contents of 

 the net at the middle depth were different, confined to an 

 abundant species of copepod loaded with ova {Euc/tcefa 

 7wr%)egica). With them there were two or three adult 

 schizopods [Nyciiphanes norvegicd)P At 70 fathoms the 

 temperature was 45°"9, at 30 fathoms 45°"6, and at the 

 position of the middle net 46°'3. Mr. Robertson con- 

 cludes : "As the middle water of the loch at this time is 

 shown to be warmer than either the layer above or below, 

 we may reasonably assume that the species in ova sought 

 the warmer layer." Similar observations repeated at 

 many different places during the March trip showed the 

 same result, the minute Crustacea being most abundant 

 where the temperature was highest. 



The work is being carried on meanwhile, purely as a 

 piece of physical and meteorological research, and a con- 

 siderable time must necessarily elapse before all the 

 latent meaning of the great mass of figures" now being 

 accumulated can be brought to light. There is no doubt 

 that when the problem of the interchanges of heat in com- 

 paratively deep water has been made out, important 

 practical applications to other sciences, and to some arts 

 and industries, will be discovered. 



Hugh Robert Mill. 



SCIENCE AND GUNNERY.^ 



II. 



T AST week we pointed out the great advantages which 

 "^-' accrue from retiring guns behind inconspicuous 

 parapets, and mentioned that the energy of the discharge 

 had been utilised to raise the guns again into the firing 

 position without the aid of extraneous power. 



^ Continued from p. 37. 



The theory of the discharge of cannon involves many 

 interesting considerations, not only with respect to the 

 strength and structure of the guns but also with reference 

 to the force required to control the recoil. A gun may be 

 considered as a heat engine of the simplest construction, 

 performing its work in one stroke. The fuel used is gun- 

 powder, and the energy developed is, as in other engines 

 of this class, in proportion to the weight of fuel used and 

 to the heat it is capable of developing. The main differ- 

 ence between explosives and most other fuels is that 

 explosives are complete in themselves ; that is to say, 

 they burn independently of the presence of extraneous 

 bodies, and that consequently the chemical union which 

 causes the explosion takes place simultaneously through- 

 out the mass and in an exceedingly short time. 



Fuel in large masses burns slowly because the air, 

 which forms its complement, can come into contact with 

 only limited surfaces, but if reduced to fine powder the 

 combustion may be made to assume almost the intensity 

 of an explosion, as for example in the dust-fuel used in 

 Crampton's furnace, and the dusty atmosphere of coal- 

 mines and flour- mills. 



The materials in gunpowder, intimately mixed through- 

 out, are in a state of unstable equilibrium with respect to 

 each other ; a very moderate increase to the thermal move- 

 ment of the molecules causes them to clash together with 

 sufficient energy to insure combination, and if such increase 

 of motion be communicated to one portion of the explosive 

 by the application of percussion or of a hot body, it is 

 carried through the mass by the luminiferous ether with 

 all the rapidity with which radiant energy travels, 

 and the increase of motion, sufficient to cause combina- 

 tion, is communicated to every molecule nearly simul- 

 taneously, the consequence being a change of form and 

 volume produced with the suddenness which marks an 

 explosion. We believe that Mr. Anderson was the first, in 

 his lectures on heat at the Society of Arts, to point out 

 that it is unfair to compare the calorific value of fuels in 

 their incomplete form ; that is to say, that such fuels as 

 require air for combustion should have the necessary 

 weight of air added to them, and when that was done the 

 singular fact appeared that the quantity of heat evolved by 

 most combustibles per unit of weight was very nearly the 

 same ; thus in nine cases cited, which included coal, coke, 

 wood, petroleum, illuminating gas, and gunpowder, the 

 extreme variations from the mean calorific value did not 

 exceed 9 per cent. In the same lectures it was shown 

 that in guns, as in most heat-engines, a very large propor- 

 tion of the thermal energy of the fuel was dissipated in a 

 useless manner ; in the case of cannon more than half 

 was wasted in heating up the gun, and about one-third 

 only in producing recoil, which was the reaction to the 

 energy communicated to the shot, to that imparted to the 

 powder gases, and to the work of displacing the atmo- 

 sphere. Of these three effects only the energy imparted 

 to the shot was known with precision, for by means of 

 sufficiently simple apparatus it was possible to determine 

 with great accuracy the velocity with which the projectile 

 left the gun, and the energy therefore was easily deter- 

 mined by multiplying half its mass by the square of that 

 velocity. 



The determination of the work done in expelling the 

 powder gases was more difficult to estimate. In the first 

 place, only about 43 per cent, of the products of the combus- 

 tion of gunpowder are in the state of gas, the remaining 57 

 per cent, are in the form of very finely- divided solids ; next, 

 the combustion goes on nearly all the time that the shot 

 is travelling out of the gun, the pebbles of powder ignit- 

 ing in succession, a fact which is proved by the circum- 

 stance that in short guns a good deal of powder is blown 

 out without being consumed at all, and it is doubtful even 

 whether in the modern long guns combustion is always 

 complete. While the shot is travelling along the chase, 

 the centre of gravity of the powder charge is moving also 



