THE ANTI-TEADES OE PASSAGE WINDS. 187 



The first is that on the one tack his barometer has a tendency to rise, on 

 the other it has a tendency to fall. The tack of rising barometei is the 

 starboard in the Northern, the port in the Southern Hemisphere. This 

 may be explained as follows : — 



According to Buys-Ballot's Law, in the Northern Hemisphere, the lower 

 barometer is on your left when your back is turned to the wind, and as 

 when you are thus placed a ship on the starboard tack is advancing towards 

 your right, she goes towards the higher barometer and recedes from the 

 lower. In the Southern Hemisphere this is reversed. But this rule will 

 only be strictly applicable so long as no change takes place in the baro- 

 metric pressure, and it may so happen that a high pressure towards which 

 the ship is going may be receding from her faster than she sails, and a 

 lower pressure may be coming up astern and overtaking her ; or it may be 

 that a lower pressure towards which the ship is sailing may be moving 

 away faster than she sails. 



Still the influence of the tack must always be felt, and on the whole it 

 may be said that in the Northern Hemisphere, a rising barometer on the 

 starboard tack is not a suflScient indication of improving weather, and other 

 signs should be looked for before trusting it. In all cases for the Northern 

 Hemisphere a rising barometer on the port tack is a valuable indication of 

 improving weather, while a falling barometer on the starboard tack is an 

 important warning in the other direction. This order is reversed in the 

 Southern Hemisphere. 



The second point to consider is the relation which the course and speed 

 of the ship bear to the tracks and progress of the areas of low barometric 

 pressure and their corresponding wind systems, in parts of the ocean where 

 the general tracks of Storms are known. 



Ships bound Westward across the Atlantic meet the advancing Storm 

 systems, and when homeward bound run with them ; consequently, the 

 rapidity with which the barometer falls or rises and the wind shifts is pro- 

 portionately greater in the former case than in the latter. 



(123.) From the same pamphlet we make the following extracts con- 

 cerning the ordinary Gales of the North Temperate Zone. 



The ordinary gales of the North Temperate Zone commence at S.E. or 

 South and end at West or N.W. The following may be cited as an illus- 

 tration of what frequently occurs to a sailing ship. A homeward-bound 

 ship, in about lat. 45° N., long. 30° W., falls in with a fresh Southerly 

 wind, and from what has been said, the captain knows that there is a lower 

 pressure to the West of him, and he may safely consider that it is travelling 

 to the Eastward ; but his ship is also going East, and his barometer may 

 remain steady, or even rise if he is outstripping the low pressure in its 

 advance. 



In the event of having to heave-to, the amount of fall in the barometer 

 per hour is a good though not certain guide ; a fall of -04 to -10 of an inch 

 per hour is usually considered to be a serious indication of the approach 

 of a Southerly gale, which may be followed by an equally fast rise, accom- 

 panied by a West or N.W. gale. 



With a Southerly wind and falling barometer, a ship bound to the West- 

 ward might gain by running to the Northward with the object of causing 



