October i8, 1917] 



NATURE 



129 



selected by Prof. Kapteyn for his " Plan of 

 Selected Areas " {1906), the number of regions for 

 which data were available for preliminary discus- 

 sion being sixty-five at Harvard and eighty- 

 eight at Mount Wilson. Dr. van Rhijn's 

 discussion is much the more detailed, Prof. 

 Scares dealing only with the numbers of 

 stars down to the limiting magnitude on 

 each plate. The two investigations agree, how- 

 ever, in indicating a progressive increase in the 

 concentration of the stars towards the galactic 

 plane, as we proceed from brighter to fainter stars. 

 Comparing star densities (a) in a belt of 20° on 

 either side of the galaxy, and (b) in the caps of 

 50° radius round the galactic poles, the ratio is 

 found to be 25 for stars brighter than 5^*0 or 

 ^™o> 5 5 for stars brighter than i6™*o (Har- 

 vard), and about 10 for those brighter than i7™'o 

 (Mount Wilson) ; magnitudes are here reckoned 

 on the revised Harvard photographic scale. 



These results differ to some extent from those 

 arrived at by earlier investigators, among whom 

 the more recent are Pickering (1903), Kapteyn 

 (1908), and Chapman and Melotte (1914). The 

 differences, however, can now, in the main, be 

 accounted for. The counts on which the first- 

 mentioned work was based were incomplete in the 

 richer regions of the sky, and the galactic con- 

 densation thus appeared to increase but little for 

 the fainter stars. Kapteyn 's memoir, on the con- 

 trary, gave values of the condensation which are 

 larger than those mentioned above, but which are 

 brought into fair agreement with them when his 

 magnitude scale is corrected to the absolute scale 

 determined more recently. Chapman and Melotte 's 

 investigation, which was photographic (thirty 

 plates), and in many respects similar to the new 

 Harvard-Groningen study, gave too small a galac- 

 tic concentration for the faint stars {i.e. 3"i at 

 i6'»'o). Dr. van Rhijn points out that the method 

 of reduction which they adopted was faulty, and 

 would lead to too small a value ; from a note in 

 his memoir it appears that, after allowing for this 

 error, Chapman and Melotte find that their data 

 yield results in close accordance with his own. 



It is of interest to note that Dr. van Rhijn's 

 estimate of the total number of stars in the sky 

 brighter than the i6th magnitude is approximately 

 thirty-three millions. Also, if the law of increase 

 in number which is obeyed down to this limit is 

 used to obtain, by extrapolation, an estimate of 

 the total number of stars of all magnitudes, the 

 result is found to be 3360 millions ; of these stars 

 about half will be brighter than magnitude 25*5. 



S. C. ^ 



ALCOHOL FUEL AND ENGINES. 

 'X'HE special committee on "Alcohol Fuel and 

 -*■ Engines" of the Australian Commonwealth 

 Advisory Committee on Science and Industry has 

 presented its first report. The need for such an 

 investigation arises from the prospective shortage 

 of supplies of mineral oils and the consequent high 

 price of motor spirit. 



NO. 2503, VOL. lOol 



There are three branches into which the inquiry 

 divides, namely, the design and manufacture of 

 the engine, the supply of the alcohol, and its 

 denaturation. As the committee points out, there 

 is no special difficulty with the engine ; indeed, the 

 use of alcohol, by permitting the compression ratio 

 to be increased considerably, offers a prospect of 

 some gain in thermal economy. Nor with a given 

 engine need there be any loss of power, since the 

 average calorific value per unit volume of a mix- 

 ture of alcohol with the air necessary for its 

 combustion is almost identical with the corre- 

 sponding figure for petrol. Nevertheless, if alco- 

 hol be compared with petrol pound for pound, 

 the latter has the advantage by some 50 per cent. 

 A comparison gallon for gallon is more favourable 

 to alcohol, but, whichever be adopted, alcohol 

 needs to be at a lower unit price than petrol to 

 be the more economical fuel. 



Alcohol unfortunately suffers from the grave 

 disadvantage that an engine will not ' ' start up from 

 the cold" with it, whereas its alternative rival, 

 benzol, will easily enable this to be done on all but 

 the very coWest days. Doubtless the possible use of 

 benzol was outside the terms of the reference of this 

 particular committee ; but, had it not been so, the 

 committee would certainly have had to point out 

 that, although the alterations to the engine to suit 

 it to alcohol as fuel are not considerable, the use of 

 benzol enables the engine to be used without any 

 alterations whatever. 



The main difficulty seen by the committee is 

 the question of supply. " It appears unlikely that 

 any considerable quantity of alcohol can be manu- 

 factured in Australia from either waste or raw 

 materials not at present utilised." The same 

 remark applied to benzol before the war, but will 

 it apply after? The quantity of benzol at 

 present being produced for munitions is shrouded 

 by the secrecy of war, but it is not out of place 

 to speculate that when the need for vast munitions 

 ceases a supply of benzol may be released for use 

 in internal-combustion motors which will give a 

 new aspect to the problem which this Australian 

 committee is considering. The further reports of 

 the committee will be looked forward to with 

 interest. 



NOTES. 



Among the several matters which the deputation 

 from the Joint Committee of Approved Societies and 

 the Conference of the Amalgamated Society of Indus- 

 trial Assurance brought before the Prime Minister, when 

 he received it on October 11, not the least important 

 was that with regard to the early establishment of a 

 Ministry of Health. Mr. Lloyd George did not fail 

 to recognise its 'mportance, and in the reply which 

 he made to the deputation showed that he was alive 

 to the necessities of the case and understood the point 

 of view of the deputation. In effect, the demand 

 made was that, in connection with the establishment 

 of the Ministry, the aim should be to give satisfaction 

 to the national insurance organisations. It was sug- 

 gested, too, Ihat the one thing that must be done was 

 to avoid allowing the Local Government Board to have 

 any part in the work. The reason for asking this 



