October 29, 1903] 



NATURE 



625 



may assume steam which is dry and just saturated to 

 behave as if it were a gas the 7 of which is 1.13, and 

 steam with 25 per cent, of moisture as if it were a gas 

 the 7 of which is 1-113. It results that the velocity in 

 the throat delivering steam is never greater than the 

 velocity of sound in such steam as exists in the throat, 

 and the pressure in the throat is never less than 58 per 

 cent, of the pressure inside the vessel, however low the 

 pressure of the outside space may be. 



Mr. Napier's experiments first directed attention to 

 this phenomenon, and Prof. Osborne Reynolds, in 

 1885 (" Collected Papers," vol. ii. p. 311), gave the 

 explanation. 



Students are still too much influenced by their know- 

 ledge of flowing water; they cannot help thinking 

 that the flow of a gas is analogous, whereas in all 

 important particulars the flow of a gas is entirely 

 different from the flow of a liquid. After much un- 

 belief among students of this subject, it is now be- 

 coming known that when there is a divergent mouth- 

 piece outside the throat, the velocity of a compressible 

 fluid may become very much greater than the velocity 

 of sound ; speeds of 3000 or 4000 feet per second seem 

 to be possible at the ends of the divergent orifices used 

 in the Laval turbine. Some years ago I framed a 

 theory of the injector which seemed reasonable, and 

 yet I found it wrong in its application to experimental 

 results. I now know that it was really a good work- 

 ing theory. It seemed to be wrong really because I 

 could not imagine a velocity of steam greater than that 

 found by Napier, the velocity of sound. 



I wish to show that the reasoning of Prof. Osborne 

 Reynolds leads to an explanation of what occurs in 

 an expanding mouthpiece. The motion is steady in 

 the vessel until the narrowest part or throat is reached ; 

 in the expanding mouthpiece the motion is turbulent, 

 but perhaps I may be allowed to consider the motion 

 as steady throughout, as this will illustrate what occurs 

 well enough, and turbulent motion mathematics is 

 quite beyond my powers. 



If W is the weight of gas passing along a stream 

 tube the cross section of which is A, then at a place 

 where the pressure is /> we know from the usual 

 reasoning that 



W: 



^Ax/y?^«'oAp'-.«")^ 



■ Awv 



if w is the weight of unit volume of the gas, being 

 Wg where p is />, and if o stands for p/p^- 



Now let us keep \V constant, and we are able to 

 calculate the cross section of the stream at any place 

 where p is known. 



I sometimes ask the individuals of a class of students 

 to calculate, each of them, a part of such a table as 

 the following : — 



Imagine steam in a vessel at />j= 14400, or 100 lb. 

 per square inch, to flow towards a throat with an ex- 

 panding orifice outside ; at the following pressures I 

 give the corresponding cross sections A of a stream 

 tube and the velocity there. It will be seen that where 

 the tube is narrowest the pressure is 57-85 lb. per square 

 inch ; this is near the narrowest part of the orifice. 

 Beyond this in the expanding part A increases, the 

 pressure falls, and the velocity becomes greater and 

 greater. 



I take a stream tube in which the flow is i lb. per 

 second, or W=i. These numbers deserve study. It 

 is evident that to get very high speeds the mouthpiece 

 must be much enlarged from the throat, but as rapid 

 enlargement must lead to greater turbulence, veloci- 

 ties much greater than 3000 feet per second ought 

 hardly to be expected. 



If we double all the pressures in the table, the values 

 of A and v there given are right for the case of flow 

 of steam from a vessel where Pg is 200 lb. per square 



NO. 1774, VOL. 68] 



inch ; about two pounds of steam per second now flows 

 along the tube. 



An expanding mouthpiece increases the flow of 

 water, and velocities are less where cross sections are 

 greater; but in the case of air or steam, the total 

 quantity flowing is not increased, and velocities are 

 greater where cross sections are greater. 



John Perrv. 



PROGRESS OF GEOLOGICAL SURVEY OF 

 THE UNITED KINGDOM. 



IT would be impossible to give on one page an 

 epitome of the work done in a year by the Geo- 

 logical Survey, but it may be possible to explain the 

 arrangement of the official summary of progress and 

 to indicate the character and range of the information 

 contained in it. 



By far the greater number of persons who consult 

 it want first of all to learn whether anything new has 

 been published about their own district. We find, 

 therefore; that the information is arranged geo- 

 graphically under the heads England and Wales, Scot- 

 land and Ireland, and that subordinate to these there 

 is a reference to districts, not well defined physical or 

 political divisions of permanent importance, but 

 divisions arbitrarily chosen for the purpose of easy 

 reference to the areas over which the work of the year 

 has been carried on. 



The descriptions are further classified under the 

 names of the geological formations found in each 

 district. 



The most important part of the work deals, of 

 course, with the observations made in the field and 

 recorded on the maps and sections, or described in 

 memoirs and explanations, but the palaeontological, 

 petrological and chemical work all receive special 

 notice, as do the products of economic value and the 

 excellent museum connected with, and largely brought 

 together by, the Survey. 



All who are engaged in geological teaching or re- 

 search, or the practical application of the science, must 

 watch the results obtained by the Survey, whether they 

 involve, as proved by Mr. Thomas, a correction of the 

 section across the Towy Valley, or throw light on the 

 relation of the Devonian to the Old Red, as rnay be 

 seen in Mr. Strahan's work, or furnish material for 

 determining the exact " geological equivalents " of 

 the coal-bearing strata in several distinct and isolated 

 areas, as shown by Mr. Kidston, or data for discuss- 

 ing with Mr. Clement Reid the conditions which pre- 

 vailed when the deposits were laid down in which 

 man's remains first appear. 



The practical man, who has always met with so 

 much courtesy and assistance in the Survey Office, 

 whether he seeks how he may find water or in which 

 direction he might hope to pick up again a lost seam 

 of coal or vein of metal, has always turned to the 

 publications of the Survey for the results of the latest 

 and most careful examination of the district in which 

 he is interested. 



