144 



SCIENCE 



[N. S. Vol. LI. No. 1310 



GRAVITY AND AEROSTATIC PRESSURE 

 ON FAST SHIPS AND AIRPLANES 



The latest issue of the Meteorological Office 

 Circular, No. 42, December 1, 1919, contains 

 an interesting note on the Behavior of Marine 

 Barometers on board fast ships. The views 

 expressed are based on certain experiments 

 made by Professor Duffield upon the value of 

 gravity at sea. In his work it became nec- 

 essary to study carefully the variations of 

 pressure recorded by a mercury barometer of 

 the new type under different conditions of 

 ship motion. 



It has been suspected for a long time that 

 on fast ships and in strong winds, pressure 

 readings might be considerably influenced by 

 eddy action. 



The experiments in this case were carried 

 out on H.M.S. Pluchy, a destroyer. Steaming 

 at 22 knots against a head wind of about 12 

 m/s., the barometer showed a fall of 1.2 kilo- 

 bars compared with the reading when going 

 with the wind. This is an aspiration efl^ect 

 and will vary with the location of the instru- 

 ment aboard the ship. Three barometers were 

 used and the change in the cabins was only 

 0.4 kb. The fall is sudden and unless the 

 navigating officer is posted might be taken as 

 an indication of impending change in weather. 

 It is stated that opening or closing doors and 

 ports did not materially affect the readings 

 but this we are disposed to question since it 

 has long been known that very noticeable 

 aerostatic pressure variations occur during 

 high winds on opening or shutting doors and 

 windows. At Blue Hill Observatory using 

 large and sensitive barographs with fast 

 moving record sheets we have obtained vari- 

 ations of from 3 to 5 kbs. The location of 

 the oi>ening determines the character of the 

 change; windward openings cause a rise, lee- 

 ward ones, a fall. 



This brings home the necessity of correcting 

 the records of fast ships and it would be 

 especially interesting if our Hydrographic 

 Office would fm-nish open scale barographs to 

 fast ships and analyze the variations in aero- 

 static pressure when such vessels were en- 

 countering high winds ahead or astern. If 



our ships and planes could also carry pressure 

 tube anemometers of the Dines's pattern or 

 the modified form provided for the Navy, 

 records showing to a nicety gustiness and rela- 

 tion of speed to pressure would be available. 



The next interesting feature of these ex- 

 periments is the deduction that a ship moving 

 east and therefore travelling with the earth's 

 rotation experiences a consequent increase in 

 the centrifugal tendency, resulting in a slight 

 decrease in the value of gravity as indicated 

 by a mercurial barometer. A west-bound ship, 

 on the other hand, would show an apparent 

 increase. This was put to test on the Pluchy 

 and it was found that 



on a west course the mercury barometer when com- 

 pared with an aneroid stood relatively higher than 

 when on an east course, indicating that the mer- 

 cury weighs less because a longer column is needed 

 to give the same pressure. For a speed of 22 knots 

 the difference amounted to approximately 0.2 kb. 



Since bodies travelling east are lighter than 

 when they are travelling west, we expect to find 

 (other things being equal) a west wind above an 

 east wind, a shell fired east with a longer range 

 than when fired west, and an airship going east 

 with a larger carrying capacity than when flying 

 west. H. M. S. Plucky weighed about 4 owt. less 

 on an east course than when steaming west. 



Professor Edward V. Huntington in Sci- 

 ence, January 9, 1920, p. 45, shows that a 

 body moving westward at high speed requires 

 an increase in the supporting force. 



Dr. Carl Herring in the same issue discusses 

 the possibility of moving a mass so rapidly 

 that the net weight would be zero. 



Aerographers of course are familiar with the 

 equation on which the above reasoning for 

 gravity rests, namely 'iwv cos <^ sin a- In this, 

 0) is the angular velocity of the earth's rota- 

 tion, that is 2^86164 seconds or .0000Y292 

 radians per second; v, the velocity of the ship 

 in meters per second, <j), the latitude and a the 

 deviation from true north or south, of the 

 ship's course. Dr. Duffield gives this value 

 for latitude 50° N. as .005 kb. per knot. 



Another matter under discussion is the 

 effect of the ship's vibration due to engines 

 upon the sensitiveness of the barograph rec- 

 ord. At present it can be said that on a vi- 



