122 MOTIONS AND PKESSUEE OF THE ATMOSPHEEB. 



simultaneously on land. It is quite clear that this is inapplicable to the 

 condition of a ship on a voyage. Still something can be learned from the 

 known phenomena of what is probable, and the reader is referred to a 

 valuable paper in the " Mercantile Marine Magazine," 1872, by Mr. Birt, 

 for many important practical hints, which are too long for quoting here, 

 but a few extracts must suflBce. They relate to an examination of the 

 Weather Charts, issued by the Meteorological Office, between March 15th 

 and April 13th, 1872, the results of which are given in a tabular form.* 



" In order to see the practical bearing of this Table (not repeated here), 

 it is necessary to recall the main feature of Buys-Ballot's Law, viz., that 

 the areas of high and low pressure are specially related to the direction of 

 the wind. Taking the direction of the wind from East to West for example, 

 and sailing or steaming with it, the area of high pressure is on the star- 

 board or North, and that of low pressure on the port or left. The steepest 

 gradient between Portsmouth and Dover during the thirty days is + •142, 

 wind at Dover North force 6. This gradient occurred on the 19th, but as 

 no return for that day was received from Scilly, the barometric section 

 from Scilly to Dover is not complete. The section for the previous day, 

 the 18th, is complete, and shows a difference of about a quarter of an inch 

 between the extreme stations. At all the stations the barometer was 

 falling between 8 a.m. of the 18th and 8 a.m. of the 19th. Suppose we 

 take a vessel on the 18th, having just passed the Straits at 8 a.m. During 

 her passage down Channel her barometer rose, wind off Dover W.N.W., 

 force 3, area of high pressure towards S.S.W. On consulting the Weather 

 Charts, we find the isobars so arranged as to indicate the anterior slope of 

 an atmospheric wave, stretching from S.S.W. or thereabout, towards 

 Norway N.N.E. ; the vessel was therefore sailing or steaming towards 

 the crest. In the case of a vessel steaming up Channel on the 19th, she 

 would have had a falling barometer with increasing wind force. 



" On selecting the steepest gradient, dip towards the West ( — ), we find 

 it to be — -071, wind South, force 3, at Dover, on March 27th, the wind 

 increasing in force Westward, being at Scilly 7, with a sea disturbance of 6, 

 marked ' rough ' on the map ; area of highest pressure to the Eastward ; 

 passage up Channel, barometer rising, wind force decreasing ; down 

 Channel, barometer faUing, with wind force increasing. The Weather 

 Chart for this day shows the isobars arranged as the posterior slop« of a 

 N.W. atmospheric wave. A vessel coming up Channel approached the 

 crest rather obliquely. 



" It must especially be borne in mind that each vessel is entirely isolated, 

 the commander having only the height of the barometer and direction of 

 the wind at his ship, with Buys-BaUot's Law, from which he is able to 

 infer in what directions the high or low pressures exist, but whether he is 

 approaching a crest or trough directly or obliquely, his data are inadequate 

 to determine. We are not aware that sufficient has been effected at land 

 stations to enable him to ascertain the general lay of the isobars from the 

 rise or fall of the barometer at his ship, and here we see the value of 

 discussing a large number of observations, in order to mark out with 



* See, also, " Weather Charts and Storm Warnings," by B. H. Scott, M.A., P.R.S. 



