264 Hon. Ralph Abercromby on the Height, 



open scale, divided to the xoo^ n mcn 5 so ^hat ^ ne readings 

 could be taken at a glance to 0*025 inch, or, when time 

 allowed, to 0*020. The instrument is an extremely good and 

 accurate barometer. The altitudes were all calculated on the 

 simple assumption that a difference of 1 foot in height is given 

 by a difference of 0*001 inch of pressure. Any error which 

 could arise between this reduction and that by a more rigorous 

 method would be far less than the other errors of observation. 

 So far the observation was simple enough ; but the great 

 difficulty arose when the height of the eye above the sea-level, 

 at the moment of observation, had to be estimated. For 

 instance, when the barometer was at its lowest point the sur- 

 face of the water might be 10 feet below the eye ; but when 

 the crest of the wave rushed past, the height might be reduced 

 to 1 or 2 feet. Here I had to trust to my own estimation by 

 eye, aided by a few rough measurements with a piece of string- 

 down the ship's side. The principal uncertainty of the results 

 depends on errors of this estimation. I do not think that the 

 aneroid errors would ever be more than 2 or 2*5 feet, while 

 those of estimation might be at least 2 feet either way. 



Length and velocity were determined by standing in a 

 suitable position with a chronograph, and measuring (1) the 

 interval between the time when two successive crests reached 

 the stern, and (2) the time that the crest of the first wave took 

 to run the length of the ship. Then the length of the ship, 

 her speed, and course relative to the direction of the wave's 

 progress being known, the velocity and length of wave could 

 be readily calculated by the following formulae : — 

 Let t = time from crest to crest, in seconds ; 



T= time of first crest running the length of the ship, 

 in seconds ; 



I = length of wave, in miles ; 



v = velocity of wave, in miles per hour ; 



k = speed of ship, in miles per hour ; 



L = length of ship, in fractions of a mile ; 



6 = angle between course of ship and direction of wave- 

 progress. 

 Then, for a following sea, 



v = L cost'— ^ H&cos0; 



l = (v-kcos6)^~ in miles, 



3600' 



t 

 b'600 



■■(v—k cos 6) .tt-^t x 5280, in feet. 



