July 17, 1890J 



NATURE 



271 



summer of each hemisphere is in perihelion or in aphelion. This, 

 in consequence of the precession of the equinoxes, will occur at 

 intervals of about 25,000 years. That is to say, if in either 

 hemisphere the summer is now in perihelion, at the end of 

 12,500 years its summer will be in aphelion, and in 12,500 

 years more it will be in perihelion again. Mr. Croll maintains 

 that glaci?lion occurs in the hemisphere where there is peri- 

 helion summer and aphelion winter, because of the intense cold 

 of such a winter. I think, on the contrary, that the facts of 

 climate which come under our observation show that winter 

 cold has little or no effect in producing glaciation ; and that a 

 cold summer, which leaves the winter snow unmelted, is the 

 most favourable condition for glaciation. Such is the climate of 

 the Antarctic continent now. It is obvious that a summer in 

 aphelion, when the eccentricity of the earth's orbit was many 

 times greater than now, must have been a very cold summer. 



This theory of the glacial climate appears perfectly satis- 

 factory. The astronomical cause is known to exist, the 

 Ideological effects are known to exist, and the effect is that 

 which the cause must necessarily produce. 



Even if it were true that a glacial climate prevailed in both 

 hemispheres at the same time, no geological evidence could 

 prove such a fact. No geological evidence could tell whether 

 glacial mounds in Norway and in Patagonia, for instance, were 

 strictly contemporary, or separated in date by; an interval of 

 12,000 years. 



D.-. Neumayr appears to retain the old notion that changes of 

 climate may be to some extent due to changes in the position of 

 the earth's poles. I am no mathematician, and cannot speak 

 on such a subject with any authority, but Sir William Thomson 

 believes he has proved that the earth is for all dynamical pur- 

 poses perfectly solid and rigid ; and I should think that the axis 

 of rotation of a perfectly rigid^oblate spheroid is unchangeable. 



Belfast, July 10. Joseph John Murphy. 



The American Meteor. 



I RECEIVED the following observations from my son, G. S. 

 Henslow, who witnessed the fall of the meteor referred to lately 

 in Nature. I forward it, as it may perhaps interest some of 

 the readers of this journal. 



"The meteor fell about 5 p.m., and divided in mid-air, part of 

 it falling in Minnesota near a town called Kasota ; this portion 

 was not found. The other and larger piece fell near Butt City, 

 Iowa. The two places are about a hundred miles distant. It 

 exploded on reaching the ground into myriads of fragments, a 

 number of which have been picked up and sold at fabulous 

 prices. The State University of Minnesota bought the largest 

 piece. It fell on the open prairie, but broke into such small 

 fragments that the surrounding soil was scarcely disturbed at all. 

 We all saw it fall here at Windom. It illuminated the southern 

 sky, and left a cloud resembling the smoke from the funnel of an 

 engine. On bursting, there was a sound like a sharp peal of 

 thunder." G. Henslow. 



SPONTANEOUS IGNITION AND EXPLOSIONS 

 IN COAL BUNKERS. 



A T the Royal United Service Institution, on Friday, 

 ■^^ July 4, a paper on this subject was read by Prof. 

 \'ivian B. Lewes, Royal Naval College. Rear-Admiral 

 N. Bowden-Smith was in the chair. 



The lecturer, after premising that in the fast ocean 

 steamers it is now becoming an event of frequent occur- 

 rence for the contents of the bunkers to spontaneously 

 i,<;nite, whilst in the Service such a thing as fire in the 

 bunkers is practically unknown, and an occasional, 

 although fortunately very rare, explosion of gas is the 

 worst trouble which the coal stores of our naval monsters 

 have given rise to, directed attention to the causes which 

 give rise to the so-called "spontaneous ignition of coals," 

 and traced the particular circumstances which tend to 

 increase the tendency to it. 



The pyrites or coal brasses present in the coal when 

 ■exposed to dry air undergo little or no change, but when 

 moisture as well as air is present they absorb oxygen and 



NO. 1 08 I, VOL. 42] 



combine with it, forming sulphates of iron, and the 

 ordinary explanation of the spontaneous ignition of coal 

 is that this process of oxidation causes a rise of tempera- 

 ture in the coal which determines its ignition ; this, how- 

 ever, has of late years been much doubted, and it can 

 now be proved that the pyrites when present in ordinary 

 quantities are perfectly incapable of doing more than 

 adding slightly to the general rise of temperature, 

 although when present in very large masses they may 

 increase the tendency of the coal to spontaneous com- 

 bustion by swelling during oxidation, and causing the 

 coal to crumble, and also by setting free sulphur, which, 

 having a lower melting-point of ignition than coal (482° F., 

 or 250° C.) would lower the temperature at which the 

 mass would catch fire. 



The real causes which give rise to heating and ignition 

 in any large accumulation of coal are twofold. First, the 

 absorption of oxygen from the air by the carbon ; and 

 secondly, the chemical action set up by the absorbed 

 oxygen with the hydrocarbons of the coal. 



The most important point to be noticed is the extra- 

 ordinary effect which initial temperature has on the 

 rapidity of chemical actions of this kind. At a low 

 temperature, and indeed up to about 100" F. = 38° C, 

 the absorption of oxygen, and consequent chemical 

 action, will go on slowly with practically little or no 

 chance of undue heating taking place, but directly the 

 temperature exceeds loo'^ F., then, with some classes of 

 coal, ignition is only a question of time and mass. 



Although the ignition point of various coals lies above 

 700^ F., yet if many of these coals are powdered, and are 

 placed in perforated zinc cases in masses of 2 lbs. or 

 upwards, and these are kept at a steady temperature of 

 about 250^ F. in an oven, ignition will generally follow in 

 a few hours ; whilst between this and 150^ F. it will take 

 days instead of hours for the same result to follow, and at 

 ordinary English temperatures several thousand tons of 

 coal would have to be stored in a very broken condition 

 before any risk of heating or ignition would ensue. In 

 considering this question with regard to coal bunkers, it 

 must be remembered that, although the considerations 

 which had to be taken note of in the case of coal-laden 

 ships still exist, yet they are considerably modified by the 

 smallness of the amount of coal carried, and by the 

 methods of loading and storage employed. 



Liability to spontaneous ignition increases with : — 



1. The mcrease in the bulk of the cargoes. — Evidence 

 given before the Royal Commission of 1875 showed that 

 in cargoes for shipments to places beyond Europe the 

 cases reported amount to j per cent, in cargoes under 

 500 tons ; in cargoes from 500 to 1000, i per cent. ; 1 000 

 to 1500, to 3-5 per cent. ; 1500 to 2000, to 4-5 per cent. ; 

 and over 2000 tons, to no less than 9 per cent. Mass 

 influences this action in two ways : — 



{a) The larger the cargo, the more non-conducting 

 material will there be between the spot at which heating 

 is taking place and the cooling influence of the outer air. 



ip) The larger the cargo the greater will be the 

 breaking-down action of the impact of coal coming down 

 the shoot upon the portions first loaded into the ship, and 

 the larger thereby the fresh surface exposed to the action 

 of the air. 



2. The ports to which ship7nents are made (26,631 ship- 

 ments to European ports in 1873, resulting in only ten 

 casualties, whilst 4485 shipments to Asia, Africa, and 

 America gave no less than sixty). — ^This startling result is 

 due to the length of time the cargo is in the vessel, the 

 absorption and oxidation being a comparatively long 

 action, but a far more active cause is the increase of 

 temperature in the tropics, which converts slow action 

 into a rapid one. 



3. The kind of coal 0/ which the cargo consists {some 

 coals being especially liable to spontaneous heating and 

 ignition). — There is great diversity of opinion on this 



