Dec. 9, 1 886] 



NA TURE 



137 



another situated in the direction of the greatest barometric slope 

 or gradient. This is expressed in hundredths of an inch per 

 fifteen nautical miles. Now, the gradient corresponding to a 

 certain force of the wind is somewhat uncertain, particularly 

 when the force <>{ the wind exceeds a whole gale, but on an 

 average a gradient of 0"02 inches in 15 miles corresponds to a 

 force of wind equal to 6 on Beaufort's scale, o 03 to 7, o'04 to 8, 

 o"o5 to 9, o'07 to 10, o'lo to II, and where the gradient is above 

 a tenth of an inch in fifteen miles it generally blows with full 

 typhoon force. In low latitudes the gradient occasionally exceeds 

 one inch in fifteen miles. 



Curved lines drawn on a map through the places from which 



the same height of the barometer (reduced) is reported, or 

 between those that report a higher and a lower barometer, are 

 called isobars. The gradient lies at a right angle to the isobar. 

 These are the most important elements in forecasting the weather. 

 Thus during the south-west monsoon the barometer as a rule 

 reads higher over Luzon than along the China coast, the gradient 

 being directed from about south-east towards north-west, indi- 

 cating southerly winds as prevailing over the China Sea according 

 to the i2-point rule. But when, as occasionally happens in the 

 typhoon season during the south-west monsoon, readings reported 

 from stations along the south-eastern coast of China are higher 

 than those reported from Luzon, the gradient is found to be 



Principal Typhi 



reversed, being directed towards south-east, thus indicating 

 northerly winds. At such times a typhoon may be expected, 

 and the probability is increased if the barometer is falling in Luzon 

 and rising slowly in Northern China and Japan, and if cirrus 

 clouds have previously been observed to come up from east or 

 north. 



in. Nearly all the typhoons appear to have their origin east 

 or south-east of the Philippine Archipelago, in a part of the 

 ocean south of the high-pressure area that covers the Northern 

 Pacific in the summer season, which part of the ocean is charac- 

 terised by high sea-surface temperature. Typhoons are some- 

 times formed in the China Sea, but then they seldom develop 

 much energy, as they usually move quickly northwards and enter 



the mainland of China or Formosa. Owing to their small 

 dimensions they are easily avoided by such ships as may fall in 

 with them. The sea-disturbance is nothing terrible, and only 

 whole gales of wind were reported in those cases that have been 

 investigated here. If, however, a typhoon of this kind passes 

 northwards up through the Formosa Channel, it soon becomes as 

 formidable as any of those that originate in the tropical Pacific. 

 We have not traced typhoons nearer to the equater than 

 about 9°. But it appears that they may possibly in some 

 cases originate nearer than that to the equator, as hurricanes 

 have been encountered in a lower latitude. 



It frequently happens that a vessel encountering a typhoon 

 in, say, 12° north latitude and 135^ longitude east of Greenwich, . 



