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KNOWLEDGE & SCIENTIFIC NEWS. 



[March, 1907. 



h will readil) be understood that the pressure on .1 

 bod} being pushed against the air, or falling vertically 

 through it, is exactly the same as il the bod) were held 

 stationary and .1 stead) current ol air, such as wind, be 

 1I1 i\ en against it. 



In considering head resistance we will first take the 

 case "I a plain- surface propelled perpendicularly to the 

 line of advance. The problem to be decided is, what 

 is the force opposing the progress ol the piano as com- 

 pared i" the speed and ana.-' This is a mosl important 

 consideration, as it is on this that all our calcula- 

 tioi s must be based. When we come to investigate the 

 pressure developed on inclined or curved surfaces, it will 

 be seen that these arc but a certain definite proportion 

 ol the pressure that would be imparted to a plane sur- 

 face nl similar area moving at right angles. 



Supposing we find that to propel a given mass at a 

 certain speed it becomes necessary to apply a steady 

 push of one pound, it is e\ ident that in order to increase 

 that speed it will be necessary to apply more force. 



Newton, by noting the time taken by spheres in fall- 

 ing from the dome of St. Paul's Cathedral, concluded 

 that the resistance of the air on the body is proportional 

 to the squari- ol' the velocity. Later experiments have 

 shown this law to be approximately true. This is to 

 be expected, since it we imagine a plane moving 

 against the air at a given rate, if its speed be 

 doubled it will strike the air twice as hard, but it will 

 also pass over double the distance in the time, and will, 

 therefore, strike twice as many particles of air, hence the 

 pressure or force required will be four times as great. 



There is, however, still some doubt as to whether 

 this law applies when the body is travelling at com- 

 paratively high velocities. Experiments made on the 

 resistance offered by the air to projectiles moving at 

 speeds ol 2,000 or 3,000 feet per second tend to prove 

 that the resistance increases in a greater ratio than at 

 rates below a hundred miles an hour (14(1 leet per 

 second). But it is only the latter that we need now be 

 concerned with. 



To get at a proper working formula for computing 

 the resistance of the air, we put P=Kv 2 ; that is, the 

 pressure in pounds per square foot equals the square 

 of the velocity in miles per hour multiplied by a certain 

 constant K, which has not yet been very exactly deter- 

 mined. 



A number of separate experiments have been made to 

 solve this point, but the results have not been in per- 

 fect agreement. It may be desirable, considering how- 

 important the results are to the subject, briefly to- re- 

 capitulate what has been done in this line, and the con- 

 clusions come to, since, as far as I know, no connected 

 account of these has hitherto been published. 



The resistance of the air was first carefully investi- 

 gated by Robins' in 174.2. His experiments were 

 chiefly directed on tin- investigation of the resistance 

 offered to bullets fired from a musket, which at that 

 time was a matter almost entirely ignored. Later on 

 he constructed a " whirling machine," consisting ol a 

 light arm rotated by means ol a weight unwinding a 

 cord wound round its support. On the end ol the arm 

 was mounted a sphere to represent a cannon ball. 



Smeaton, who had been investigating the lone ob- 

 tainable by means of windmills, shortly afterwards pub- 

 lished a table of wind pressures! which had been 

 communicated to him by Rouse. This was compiled on 

 the supposition that K=.oo5. The table, though pro- 



■ "New Prmciplesof Gunnery." by Benjamin Robins, F.R.S.. 

 new edition, to which is added " Subsequent Tracts." 1S05. 

 I '• Philosophical Transartions " for 1759, p. 165. 



duoed m so uncertain a way, nevertheless was accepted 

 as authoritative, and is often quoted intact to this day 

 in hooks dealing with engineering and wind elicits. 

 Subsequent investigations, however, show that these 



deductions were somewhat misleading. 



In [809 Sir George Cayley came to rather different 

 conclusions as the result of his own experiments. 'I he 

 latter were conducted with an apparatus in which a 

 surface ol one square foot was mounted upon an arm 

 about five feet long androtatedb) weights over a pulley. 

 lie found after " many carefully repealed experiments, 

 that a velocity of 11.538 feet per second generated a 

 resistance of 4 ounces, and that a velocity ol 17.10 feet 

 per second gave 8 ounces resistance," which would give 

 a value of .004 and .0034 respectively lor the symbol K. 



Dr. Mutton 1 continued the experiments ol Robins, 

 using the same or a precisely similar machine (which is 

 still preserved in the model room at the Royal Academy, 

 Woolwich). His investigations were more extensive 

 and precise, and will again be referred to. 



In compiling a table of wind pressures his figures 

 differed slightly from Smeaton's table, giving less force 

 for a given velocity. In these K would work out at 

 just about .004. 



More recent investigations, which tend to prove a 

 still lower value for K, must be deferred to another 

 article. 



*„* In the last article (February Knowledge) the title of 

 Mr. Wilde's paper " On Aerial Locomotion " was wrongly printed. 



I To be continued. 1 



CORRESPONDENCE. 



To the Editors of " KNOWLEDGE & Si [ENTI1 u Xi.ws." 

 Sirs, — I have two thermometers in the same screen, one 

 wet bulb, the other dry. Usually there an- from o° to io° 

 difference between them, 'this morning the wet bulb was 

 the highest by about 2° or 3 . 



I have nut iced the same effect at other times, when the 



water in which the cotton is soaked was frozen. Why is it? 



Also, is there usually a wave of high barometric pressure 



about January 25? I see, in 1904, that there was a record 



high barometer, or something like it, then. 



Yours faithfully, 

 Hopesay Rectory, R. J. ROBERTS. 



Aston-on-Clun, S.O. 

 February 7, 1907. 

 [When the temperature is below freezing-point the wet 

 bulb thermometer requires special attention, as the muslin 

 and conducting thread become frozen and there is con- 

 sequently no supply of water to the bulb. On the occasion 

 referred to above the moisture on the muslin round the bulb 

 was evidently just passing from the liquid to the solid state, 

 and so was at the freezing-point temperature, viz., 32". The 

 mercurv in the wet bulb thermometer would remain at 32 

 until the freezing process was complete. On many occa- 

 sions — especially in rather damp weather and with the tem- 

 perature of the air not much below tin freezing point — the 

 wet bulb thermometer will remain at 32 lor a considerable 

 time — often for hours— and so be much higher than the dry 

 bulb. During frost tin muslin should be wetted about an 

 hour before the time of observation, so that a coating of ice 

 may be formed round the bulb. With regard to barometric 

 pressure, this is much more variable in winter than in 

 summer, and so in om year il may 1» very high and in 

 another year very low about the same date. .Mr. Roberts 

 mentions January 25, 1904, as the date whin a high baro- 

 metric pressure occurred. Airainst that may be placed 

 January 26, 18S4, when practically the lowest barometric 

 pressure ever recorded in the British Isles was observed, 

 viz., 27.332 ins. at Ochtertyre, near Crieff.- W. M.| 



• Nicholson' s Journal, November, 1S09. 

 Mathematical Tracts," Vol. 3. by Dr. C. Hutton. 



1 8 1 2 . 



