BAEOMETRIC GRADIENTS. 123 



approximate precision the barometric affections of such a line as the 

 South Coast of England. To effect this the labour is considerable, and it 

 can only be accomplished on the large scale by an ofl&ce charged not only 

 with the work of collecting data, but of utilizing such data when obtained. 



" In tabulating this, we have the whole hne from Dover to Scilly divided 

 into three portions, Dover to Portsmouth, Portsmouth to Plymouth, and 

 Plymouth to Scilly. So far as the most important feature — the barometric 

 gradients — is concerned, each portion differs, more or less, from its neigh- 

 bours. Taking the gradients facing the East (+), we have on the line 

 Portsmouth to Dover these gradients occurring on 20 days. On the line 

 Plymouth to Portsmouth 12 days, and from Scilly to Plymouth 15 days. 

 Of the reverse or facing the West ( — ), Dover to Portsmouth 7 days, 

 Portsmouth to Plymouth 13 days, and Plymouth to Scilly 12 days. On 

 three occasions the barometer was equal in height at Dover and Ports- 

 mouth, on five at Portsmouth and Plymouth, and on two at Plymouth and 

 Scilly. These results clearly show that the barometric movements are 

 very complex ; nevertheless, it is possible that each portion of the Channel 

 may have its specific type. 



" A graphical representation is conclusive as to the connection between 

 barometric differences or gradients, wind force, and sea disturbance. A 

 determination of the law of this connection is important. We can, how- 

 ever, look at this matter in another light ; for instance, we can take the 

 mean wind force and sea disturbance for each station as under : — 



Dover 2-7 Wind. 2-0 Sea. 



Portsmouth 3"5 ,, 30 „ 



Plymouth . . 2-4 Wind. 1-5 Sea. 

 SciUy .... 4-7 „ 4-4 „ 



" These numbers show that, with the exception of Plymouth, both wind 

 force and sea disturbance increase as we proceed Westward. Bearing in 

 mind that the scale of wind force is — 12, and that of sea disturbance — 9, 

 the increase of sea disturbance is greater in proportion at Scilly than within 

 the Channel. 



" We have already alluded to the isolated condition of each vessel as she 

 sails or steams up or down Channel ; and from the above example of dis- 

 cussing the daily readings, it appears that in the passage down Channel a 

 commander, on the average, meets with increasing force of wind, increasing 

 sea disturbance, and increasing barometric gradients. An important 

 question here arises : How is a commander to ascertain his gradient in 

 his isolated position ? On passing the Straits, his barometer stands at a 

 certain height, on passing Portsmouth it has risen, but the difference will 

 not give him his gradient, for he is ignorant of the height of the mercury 

 at Dover when he is off Portsmouth. The rise he records is compounded 

 of two quantities, that due to the gradient he has sailed through plus (-}-) 

 the absolute rise of the barometer at the two stations during his voya^^e 

 from one to the other ; or it may be that the portion due to the gradient 

 is reduced by a fall of the barometer during the interval. For example, the 

 gradient on March 19th, between Portsmouth and Dover, was + -142. On 

 the 18th the barometer stood at 30-01 at Portsmouth, on the 19th it was 

 29-94, fall in the 24 hours 0-07. On the 18th, at Dover, the barometer 



