September 15, 1923] 



NA TURE 



393 



The agreement between the observed and computed 

 values can be seen from the table below. It is suffi- 

 ciently close to suggest, in view of the approximate 

 character of the band formula used, that there may be 

 some reality in this grouping. As usual the intensity is 

 transferred from the lines with low quantum numbers 

 to those with high increase in excitation. It will be 

 noted that the line N^ is used in this grouping as well 

 as in No. 2, and the suggestion is that this line is a 

 close double of a strong and weak component, the 

 latter of which belongs to the present group. 



To summarise, of 34 lines in the nebular spectrum, 

 17, including the strongest, have been arranged in 

 suspected band groups. Without additional evidence, 

 however, no conclusion can be safely drawn as to 

 the reality of these groups. While the numerical 

 agreements are not unsatisfactory, it is far from 

 impossible that such coincidences are fortuitous. 

 Confirmation would be lent to this scheme if new 

 lines could be found which would fall into one or 

 other of the above bands ; exposures of nebular 

 spectra have been made here of as long as twenty 

 hours without, however, detecting any new lines. 

 In the meantime, then, until further evidence is forth- 

 coming, the reality of the above groups must remain 

 in doubt, and the only conclusion that may be safely 

 drawn is that there is no inherent difficulty in sup- 

 posing the nebular spectrum to have its origin in a 

 molecular carrier. 



Nature of the Hypothetical Molecule. — As the 

 general appearance of the spectrum and the separa- 

 tion of the suspected band lines suggest, the moment 

 of inertia of the hypothetical molecule must be 

 small (of the order of 2 x lo"** gm. cm.*). So 

 small a moment of inertia clearly suggests that the 

 atoms which constitute the molecule must be of 

 small mass. Of the elements hydrogen, helium, 

 carbon, and nitrogen known to exist in nebulae, only 

 atoms of the first two are, therefore, likefy to form 

 the hypothetical molecule. The spectra of the Hg 

 ■ uid Hcg molecules are already known, and there is 

 ') similarity between either of these spectra on 

 ite hand and the nebular spectrum on the other. 

 \s a working hypothesis the suggestion may there- 

 tre be made that the nebular spectrum has its origin 

 1 a H He molecule with a moment of inertia of the 

 -rder of 2 x 10"" gm. cm.*, and a resultant separa- 

 tion of the H and He nuclei of about o-i x 10"* cm. 

 In view of the known chemical activity of atomic 

 hydrogen and also of the existence of molecular 

 helium, it is not improbable that such molecules must 

 occasionally be formed. In fact, Aston (" Isotopes," 

 p. 99) has suspected their existence in his positive 

 ray experiments. 



Probably the chief merit in the foregoing dis- 

 cussion is that it furnishes a suggestive working 

 hypothesis for finding the nebular lines in the labora- 

 tory. The problem becomes one, not of finding new 

 elements — a difficult matter — but of examining the 

 spectrum of a molecule which is known to exist. 



NO. 281 I, VOL. 112] 



I While our knowledge of physical conditions in the 

 nebulae is still obscure, j'-et it may serve as a guide 

 to experimental investigation. Clearly atomic hydro- 

 gen and helium must be present in a highly rarefied 

 condition and presumably at low temperatures ; 

 such a condition can be duplicated probably by the 

 introduction of some helium in the centre of a long 

 Wood vacuum tube where atomic hydrogen is known 

 to exist in abundant quantities. Not only must the 

 conditions be right for the formation of the molecule, 

 but once formed it must be excited to radiation ; 

 for a nebular absorption spectrum is not known to 

 exist, and hence the normal hypothetical molecule 

 must radiate in the far ultra-violet. Such difficult 

 problems of laboratory technique must be left to 

 others ; an attempt, however, will be made here to 

 secure further astronomical evidence on the reality 

 or otherwise of the suspected bands. 



H. H. Plaskett. 

 Dominion Astrophysical Observatory, 

 Victoria, August 4. 



Dutch Pendulum Observations in Submarines. 



Three submarines of the Dutch Royal Navy with 

 the mother-ship Pelikaan are about to sail for Java. 

 At the request of the Dutch Geodetical Committee 

 (Rijkscommissie voor Graadmeting en Waterpassing) , 

 his Excellency the Minister of Marine has allowed 

 Dr. F. A. Vening Meinesz, engineer appointed to the 

 Committee, to join one of the submarines for the 

 purpose of making pendulum observations on board 

 during the voyage. 



For several years Dr. Vening Meinesz has been 

 engaged in determining the intensity of gravity at 

 51 stations in the Netherlands. The difficulties 

 caused by the extreme mobility of the soil in part of 

 the country induced him to work out a method for the 

 elimination of the resulting disturbances ; this has 

 been applied with complete success, as will be shown 

 in a publication — in French — to appear shortly. It 

 was hoped that the extended theory might be applied 

 to pendulum observations on board an ocean steamer. 

 A first trial, however, on a steamer of the Koninklijke 

 Paketvaart Maatschappij from Ymuiden to Flushing 

 failed through the very turbulent sea. 



In the spring of this year. Dr. Vening Meinesz gave 

 a short exposition of his theory at the Physical and 

 Medical Congress at Maastricht. Prof. F. K. Th. van 

 Itterson, director of the Government mines at Heerlen, 

 suggested that the observations might be successfully 

 carried out on board a submerged submarine, where 

 the disturbances could be expected to be less than 

 on the surface of the sea. His opinion was found to 

 be correct at a trial on board a submarine at the 

 Helder. Notwithstanding the fact that a heavy gale 

 was blowing and the sea was very rough, the move- 

 ments of the ship, submerged at a depth of 15 metres, 

 were so trifling that the amplitude of the pendulums, 

 which were hanging quietly at first, amounted to no 

 more than 8'- 12' after a quarter of an hour. 



A brief exposition of the theory as given by Dr. 

 Vening Meinesz at the Congress at Maastricht and 

 published in de Ingenieur, 1923, No. 18, may be of 

 interest. 



The influence of the horizontal and vertical move- 

 ments of the ship may be eliminated by the use of 

 two pairs of pendulums swinging together from the 

 same support, the two pairs moving in two planes. 

 In the Von Sterneck apparatus used by Dr. Vening 

 Meinesz, these two planes are at right angles to each 

 other. The movements of each pendulum are to be 

 photographically recorded. 



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