4 88 



NA TURE 



[March 26, 190^ 



air were studied, and the changes which they under- 

 went when they encountered obstacles. 



The apparatus for investigating- these movements in 

 air was of simple character ; it consisted of a chimney of 

 prismatic form (side 0-50 m., height 075 m.). The front 

 side was made of clear glass, and the posterior wall 

 was covered with black velvet ; the left wall was white 

 and the right one was glazed. 



In front of the apparatus a lantern was placed within 

 which a magnesium flash could be fired. A draught 

 was maintained through the chamber by an electric fan. 

 The flow of air was rendered steady by being filtered 

 through silk gauze of fine mesh, placed at the top and 

 at the bottom of the prismatic chamber. By a beautiful 

 method M. Marey rendered the direction of currents 

 of air visible; he introduced minute streams of smoke, 

 which were drawn in with the aspirated air, and re- 

 mained parallel to each other during their passage 

 through the chamber when not opposed by any ob- 

 stacle. The smoke was obtained from the combustion of 

 tinder and cotton in a closed furnace ; from this furnace 

 the smoke was conducted to a series of narrow tubes 

 parallel to one another. 



When an obstacle was placed within the chamber 

 the stream lines were seen to bend against the obstacle 

 and divide into two currents, one of which flowed up 

 the slope of the inclined plane, the other down it 

 (Fig. 1). The division appeared to take place at a 

 point which corresponded with the centre of pressure 

 against the inclined plane. This point of separation 

 was found to be at the middle point of the plane when 

 the plane was horizontal, and to approach its upper end 



the more the plane was inclined. Behind the obstacle 

 eddies were seen to form. 



M. Marey found the velocity of the air streams, thus. 

 By means of an electric vibrator he imparted Vibrations 

 to the smoke jet tubes, having a period of ten per 

 second. The smoke streams then became sinusoidal 

 in shape, the inflections being maintained during the 

 whole length traversed by the smoke. The series of 

 lateral inflections was measured by means of a divided 

 scale placed in the same plane as the streams of smoke. 



These inflections remained equidistant when the 

 speed of the current remained constant, but when the 

 speed was reduced the inflections were closer together, 

 and farther apart when the speed of the streams was 

 increased. M. Marey employed the magnesium flash 

 to obtain his photograph ; probably sharper pictures 

 would have been obtained by using the electric spark 

 from a charged Leyden jar as an illuminant. 1 M. 

 Marey mentions that an important question to be 



1 Spark photography of objects i"* rap'd movement (STinke \tt'^ ami smoke 

 rings photographed in collision), Junior Scientific Club, Oxflrd : NatURK, 

 vol xlvii. p. 1 19. 



answered in the science of aerial flight is, How do air 

 currents behave when passing through adjacent parallel 

 planes inclined at an angle to the stream? Fig. 2 

 answers the question clearly. The picture will suggest 

 much to those engaged in the designing of kites of 

 the box type, where the air strikes against more than 

 one plane. 



The conditions of stream line flow round different 

 aquatic animals have received considerable attention, 

 and we know that a blunt head and a pointed tail is 

 a favourable arrangement. By immersing solid bodies 

 having one end obtuse and the other pointed, it is ob- 

 servable that there is a great advantage in presenting 

 the large end to the direction of motion ; this minimises 

 the motion of the air behind the body. The same 

 phenomenon is to be seen in air. Fig. 3 shows that, 

 with the large end facing the stream, the disturbance 

 in the rear of the object is slight, only small eddies 

 being set up. M. Marey's methods are applicable 

 to an almost endless variety of similar experiments on 

 the stream lines of air round differently shaped bodies. 

 M. Marey's paper is short and condensed, but it con- 

 tains matter of much importance, and is another ex- 

 ample of the beautiful results obtained by this master 

 of experimental methods in chronography. 



"F. J. J.-S. 



THE VENTILATION OF THE TUBES. 

 TN October, 1901, the London County Council deter- 

 *■ mined to investigate the condition of the atmo- 

 sphere in the tube of the Central London Railway, in 

 order to ascertain how far the 

 threatened multiplication of under- 

 ground tubes might affect the 

 public health. As the result of 

 this, the chemist to the County 

 Council, in conjunction with Dr. 

 Andrewes, made a chemical and 

 bacteriological examination of the 

 condition of the atmosphere in the 

 tunnels, stations, carriages, and 

 lifts of the Central London Rail- 

 way, as compared with the outside 

 air under ordinary conditions. As 

 might have been expected, it was 

 shown by the experiments that the 

 fluctuations in the amount of carbon 

 dioxide and organic matter present 

 in the tube were very great. 



Examination in the early morn- 

 ing showed that the ventilation employed had produced 

 a very fair condition of air, whilst during the hours 

 of traffic the carbon dioxide rose to considerably higher 

 limits than existed in the outer atmosphere. The 

 County Council chemist considers that samples of air 

 taken at any point on the railway should not contain 

 more than double the amount which is found in the 

 air of the streets, inasmuch as the additional carbon 

 dioxide found in the air of the tunnels has been entirely 

 produced by respiration, and is therefore accompanied 

 bv organic matter. 



This report was submitted to the Council on February 

 17, but its reception was postponed, as it is clearly one 

 of those cases in which extreme caution should be used 

 in arriving at conclusions, and introducing rules and 

 ri gulations which might hamper important develop- 

 ments in the relief of our over-congested traffic. 



11m- normal quantity of carbon dioxide present in the 

 air is a little under four volumes in ten thousand, and 

 the sanitary limit, which is universally adopted for the 

 atmosphere in our dwelling-houses, is six parts in ten 

 thousand in rooms which are to be inhabited for any 



NO. 1743, VOL. 67] 



