6o 



NATURE 



[May 19, 1887 



In the hydro-pneumatic system the fall of the gun 

 actuates a ram or piston working in a cylinder full of 

 water, and communicating by an automatic valve, opening 

 outwards from the cylinder, with an air-vessel about two 

 and a half times the capacity of the ram, and filled with 

 air compressed to a degree sufficient not only to support 

 the weight of the gun, but also to raise it quickly into the 

 firing position. When the gun is up, the air-vessel is 

 nearly empty ; when down, a volume of water equal to 

 that of the ram displaces the air and increases its pres- 

 sure, and the ratio of the fall of the gun to the stroke of 

 the ram, and the relative velocities of the two, are so ad- 

 usted that the increase of air-pressure corresponds to the 

 increasing leverage which the gun acquires as it descends. 

 It is possible to provide sufficient air-pressure not only 

 to arrest the fall of the gun, but also to absorb the energy 

 of recoil ; but unless the gun is allowed to fall a very 

 great distance this is not necessary, and any excess 

 energy can be more conveniently absorbed by regulating 

 the opening of the recoil-valve so as to throttle the water 

 in its passage from the cylinder into the air-vessel. At 

 first sight it might be assumed that, saving friction of the 

 mechanism, the air-pressure which would suffice to check 

 the fall of the gun would be sufficient to raise it again ; 

 but a little consideration will show that this is not the case. 

 To allow the gun to fall in the short space of time during 

 which recoil takes place, the pressure of the air must be 

 less than that necessary to support the gun, because its 

 pressure rises nearly according to the ordinates of an 

 adiabatic curve, the temperature rising in exact propor- 

 tion to the work done. During the time the gun is being 

 loaded, the heat developed in the air is dissipated, so 

 that when the gun requires to be raised the store of heat 

 is gone, and the air, expanding, falls in temperature by 

 the amount of heat converted into the work of raising 

 the gun ; the pressure consequently falls. 



To meet these two sources of loss, amounting to the 

 heat corresponding to the work of the gun falling twice 

 the height to which it rises and falls, the energy of the 

 discharge has to be drawn upon ; it compresses the air far 

 above its isothermal line, although that line is so fixed as 

 to yield sufficient heat for conversion into the work of 

 raising the gun. In addition, the energy of discharge has 

 to provide the means of overcoming the friction of the 

 machinery which resists the falling of the gun, and again 

 resists its rising, so that, taking all the sources of loss 

 enumerated together, the energy of recoil of even our 

 most powerful guns is not adequate to do more than 

 allow them to fall some 8 or 9 feet, an amount, however, 

 sufficient for the most ample protection. 



It will be readily seen that the construction of a dis- 

 appearing carriage offers a number of problems of great 

 scientific as well as practical interest. We have only 

 dwelt upon some of the most prominent points. There 

 remam the strains on the elevating gear, which is 

 arranged so as to bring the gun into the same loading- 

 position, irrespective of the angle at which it is fired, and 

 has, therefore, to communicate a sudden rotatory motion 

 to the gun ; the resistance of the levers and elevating- 

 bars to the cross strains caused by their own inertia when 

 brought into sudden motion sideways ; the resistance 

 offered to the water in its passage at variable velocities 

 from the cylinder to the air-vessel, the accelerating force 

 to be provided to raise the gun in a given time, and many 

 minor problems which tax to the full the application of 

 mathematics to the design of machinery 



THE TOTAL SOLAR ECLIPSE OF 



AUGUST 19, 1887. 



'pHE total eclipse of the sun which will occur on 



August 19 next, though only of average duration, 



will offer exceptional opportunities for observation from 



the circumstance that the track of the moon's shadow 



will be almost entirely a continental one, in striking con- 

 trast to the eclipses of the last four years, in all of which 

 the shadow has followed a course which has been princi- 

 pally over the great oceans. The eclipse is technically a 

 partial one for the principal part of Great Britain, but as 

 it will be nearly over before sunrise, it will practically not 

 be visible here. The middle phase will have been reached 

 at sunrise, for places a little to the west of Berlin : and this 

 city lying within the path of the shadow, it is just possible 

 that it may be favoured with a sight of the phenomena of 

 totality, though with a sun close to the horizon ; for the 

 sun will be largely obscured as it rises, and will not be 

 quite 3° high at the end of the total phase. From Prussia 

 the shadow track passes into Russia, and the central line 

 does not leave the borders of the Russian Empire until it 

 reaches East longitude 112°. It then crosses Manchuria 

 and the Sea of Japan, and cuts the principal island of the 

 Japanese group a little to the north of the capital. The 

 final portion of its course lies over the North Pacific 

 Ocean, and except for the little island of Rico de Oro, it 

 does not touch land again. But the path of totality not 

 only lies mainly over land, a large number of important 

 towns are either actually included within, or lie very close 

 to its limits. Konigsberg lies just outside. Kovno, Wilna, 

 and Vitebsk, are well within the shadow ; Wilna being 

 nearly on the central line. At these towns, however, the 

 sun will still be too low for them to afford desirable 

 stations for observations, and probably the neighbourhood 

 of Moscow will be the nearest district which will be occu- 

 pied by astronomers. At Moscow itself, the eclipse will 

 not be quite total, since that city lies just outside the 

 southern edge of the shadow-track, but three lines of rail- 

 way radiating from Moscow will afford easy access to 

 places actually on the central line. The most westerly of 

 these three railways is that which unites St. Petersburg with 

 the older capital, and which passes through Twer. Twer 

 is nearly on the central line, but a little to the north of it. 

 The sun will have an elevation of about 16^ in this neigh- 

 bourhood, and the maximum duration of totality is not 

 quite two minutes and a half At Twer itself it will be 

 only 124 seconds. Three parties, two German, and one 

 French, will take up positions within the Govern- 

 ment of which Twer is the capital. The second 

 line runs from Moscow to Vologda, passing through 

 Jaroslavl, which lies within but near the edge of the 

 shadow. Petrowsk on this railway is very near the 

 central line, and here the sun will be 2° higher than near 

 Twer, and the duration 152 seconds. The third line runs 

 to Kineshma, which is itself very near the central hne. 

 Here the sun will be about 20° high, and the total 

 eclipse on the central line will last 156 seconds. It will 

 not, however, be difficult to proceed to yet more favour- 

 able positions further east. From Moscow there is a line 

 through Nijni Novgorod to Kazan, and a service of river 

 steamers runs thence up the River Kama to Perm. Perm 

 lies to the south of the central line, but the totality lasts 

 there 173 seconds, whilst the sun is 28° high at mid eclipse. 

 If the weather should be favourable, Perm would be there- 

 fore a very suitable station for those astronomers who can 

 spare the time to journey so far ; for others the neigh- 

 bourhoods of Petrowsk and Kineshma will afford readily 

 accessible sites. Prof Bredichin, Director of the Moscow- 

 Observatory, has his own private observatory only two 

 kilometres from Kineshma, and very close to the central 

 line ; and he has generously offered the hospitahty of his 

 house to the Royal Astronomical Society for two English 

 astronomers, an offer which has been gratefully accepted 

 by the Society, on behalf of Dr. Copeland and the Rev. 

 S. J. Perry. Prof C. A. Young also will have his station 

 here, and a strong party of Italian and English astro- 

 nomers, consisting of Profs. Tacchini and Ricco, and 

 Messrs. Common and Turner, will be located at no great 

 distance away, in the neighbouring Government of 

 Vladimir. 



