January 8, 1920] 



NATURE 



479 



OUR ASTRONOMICAL COLUMN. 



The Birth of the Moon. — Prof. W. H. Pickering 

 in Popular Astronomy (October, 1919) endeavours to 

 reconcile Sir George Darwin's estimate of tlie moon's 

 age (less than 60,000,000 years) with recent geological 

 opinion, which demands a period of 1,200,000,000 

 years since the formation of the earth's crust. He 

 suggests that the matter of the moon left the earth at 

 that remote epoch, but remained for ages circulating 

 round the earth as a cloud of fragments. In this 

 form its tidal influence would be small, and the earth 

 would for long^ retain its assumed primitive rotation 

 period of some four hours. Gravity in the tropics 

 would be much reduced by centrifugal force. Prof. 

 Pickering seeks thus to' explain the existence of the 

 huge reptiles like the Atlantosaurus and the Diplodocus, 

 also the fact that heavy reptiles like the Pterodactvis 

 had the power of flight. He suggests that the moon 

 was consolidated from the cloud of fragments in the 

 middle of the Cretaceous period, and quotes geological 

 authorities for a great invasion of land areas by the sea 

 and tremendous volcanic activity at that epoch, which 

 he ascribes to the great tides which the moon would 

 have raised when so near the earth. That epoch 

 would agree well enough with Sir George Darwin's 

 estimate of the moon's age, supposing it to date from 

 its consolidation, not from its leaving the earth. 



Distances ok the St.^rs of Type F. — Mr. C. F. 

 Lundahl discusses the distances of these stars in 

 Meddcl. Lunds Astr. Obs. (series ii., No. 21). He 

 works on the same lines as Prof. Charlier in his 

 recent memoir of the B stars; that is, he assumes a 

 constant absolute magnitude, and deduces, the dis- 

 tance of each star from its apparent magnitude. The 

 F stars have a wider range of absolute magnitude 

 than those of type B, but the great majority of them 

 are included in a range of about 2 mag. ; hence toler- 

 able results for the distances may be expected. In 

 fact, he states that 60 per cent, of the stars the 

 parallax of which has been measured agree with his 

 values within the limits of probable error. He quotes 

 Prof. Plummer's research on the same stars, which 

 was based on the assumption that they were moving 

 parallel to the galactic plane. .\s that method showed 

 an agreement with measured values for onlv 40 per 

 cent, of the stars, Mr. Lundahl concludes that his own 

 assumption is nearer the truth. He notes from his 

 results that E^Tucanae and »; Cassiopeiae are evldentlv 

 dwarf stars, while Polaris and still more Canopus are 

 notable giants. The density of distribution of F stars 

 is estimated by two indeoendent methods, which give 

 respectively 8 and 4 cubic siriometers for one star of 

 this type (a sir. = 1,000,000 astr. units). 



Absolute Magnitude as a Function of Colour. — 

 Mr. F. H. Seares indicates a relation between colour 

 and absolute magnitude in stars of the same Sf)ectral 

 tvpe (Proc. Nat. Acad. Sci., July, iqiq). The colour 

 is determined photographically bv taking graduated 

 exposures of the star on an isochromatic olate with 

 and without a yellow filter. The ratio of exposure 

 times that give images of equal intensity in the two 

 cases is a measure of the colour. The method has 

 been tested on about i?o stars the absolute magnitude 

 of which has been otherwise determined. The fol- 

 lowing are the results deduced : — Giant stars of tvpes 

 G and K are decidediv redder than dwarfs : also in 

 type B the brighter stars are redder, but the difference 

 is less marked. On the other hand, in tvpe A the 

 fainter stars are redder, while in tvnes F and M the 

 curve is too flat to permit of the absolute magnitude 

 being found from the colour. Thus the method can 

 be applied onlv if the spectral tvpe is known, but it 



NO. 2619, VOL. 104] 



promises to be a useful supplement to the spectroscopic 

 method. Experiments are being made to examine 

 whether the necessity of knowledge of the spectral 

 type can be evaded by taking three series of exposures 

 with screens of difi^erent colours ; if this were possible, 

 the method could be applied to much fainter stars. 



CROSS-CIRCULATION AS A PHYSIO- 

 LOGICAL METHOD. 



I N the mutual co-ordination and integration of the 

 ■*■ physiological processes in a complex organism, in 

 which actions exerted by the environment on a par- 

 ticular part affect the whole and the functional activity 

 of one organ has its influence on numerous others, 

 there are two chief methods adopted. One is by 

 means of the central nervous system, in which mes- 

 sages received from the periphery along certain nerve- 

 fibres are reflected back, as it were, to outgoing nerve- 

 fibres, setting into play the appropriate muscular or 

 other response, it may be in a distant part of the 

 organism. This method has been compared to a 

 telephone exchange. The other is by means of the 

 blood. Owing to the continual circulation of the same 

 mass of liquid through all parts of the body, it will 

 readily be seen that a chemical substance, produced in 

 any one part and passing into the blood-vessels supply- 

 ing this part, must be carried, sooner or later, to all 

 other parts, and give rise to effects in any tissue or 

 organ sensitive to it. We have here an actual trans- 

 port of material, the materials carried, when thev 

 result in changes in distant organs, being known as 

 "chemical messengers" or "hormones." 



In many cases there is difficulty in discovering to 

 which of these modes of communication a particular 

 reaction is due. Thus when muscular exercise is 

 taken, the depth and rate of breathing increase. We 

 know that carbon dioxide is produced in the combus- 

 tion process that affords the energy for the muscular 

 work. This passes into the blood, and may be in 

 itself sufficient to set into greater activity the nerve- 

 centre controlling the muscles of respiration. On the 

 other hand, it might be that sensory nerves in the 

 muscle are stimulated bv the movements, and that 

 the appropriate message is conveyed by nervous 

 channels, or both chemical and nervous factors may 

 be involved. Perhaps a clearer case is that of the 

 pancreas, which pours its powerful digestive juice into 

 the small intestine as the food arrives there from the 

 stomach. We know now that the chief, if not the 

 only, way by which this co-ordination is effected is 

 that the acid of the gastric contents causes the forma- 

 tion of a chemical messenger in the walls of the intes- 

 tine. This, passing into the blood, ultimately reaches 

 the pancreas and excites it to activity, but it was 

 for a long time believed to be a nervous reflex. Again, 

 the origin of wound-shock has recently been shown 

 to lie mainly in the production in the injured tissues 

 of poisonous compounds, which are carried bv the 

 blood to the rest of the body and cause widespread 

 damage to the capillary blood-vessels, resulting in a 

 failure of blood-supply throughout the body. At the 

 same time the co-operation of nervous factors has not 

 been altogether excluded. 



The analysis of manv problems of this kind has 

 been greatlv assisted by the various methods known 

 as "cross-circulation." It is obvious that if we can 

 make a connection between the blood-vessels of one 

 animal (A) and those of another (B), any chemical 

 messenger produced in A must affect B also, whereas 

 a process in A brought about entirely by fhe nervous 

 system will have no effect on B. In this mode of 

 experiment the blood of A may either be allowed to 

 circulate through the whole of B, and vice versa, or 



