22 REPOKT — 18G7. 



influence of tlie earth's rotation and tlie liipli temperature which prevails in the 

 northern hemisphere. At JNiaiiritius these gales are characterised by a barometric 

 pressure of aO-200 to :30'400 inches. The wind sets in at .South to 8.8.E., and 

 seldom veers more than a point or two, the barometer at times oscillating- during 

 the height of the gale, which is sometimes attended with passing showers, but 

 never with heavy rain, thunder, or lightning. Generally the gale commences in 

 about 30° S., and advances towards the equator, like an extensive wave or billow, 

 the barometer rising at each successive locality some time before the wind acquires 

 much force. It is preceded by a heavy sea, which occasionally proves dangerous 

 near the equator. It lasts from one to ten days, and blows in fitful gusts, which 

 at JMauritius have usually a pressure of 1 to 10 lbs. on the square foot, and at times 

 of 10 to 20 lbs. Owing to the frequency of these gales the mean daily maximum 

 force of the wind at Mauritius is greater in winter than in summer. 



2. The extra-tropical gales, between the parallels of 30° and 45° S., also 

 occur in all seasons, but are most violent from May to August inclusive. These 

 gales are generally characterised by the presence of two currents of air, the one 

 from the southward, and the other from the northward, the two currents being 

 variously situated with respect to each other. At times they exist side by side, as 

 surface-winds, the one fi-om the S.W., and the other from the N.E., each occupy- 

 ing a belt of 5° to 30° in longitude, stretching from the parallel of 30'^ S. as far south 

 as the observations extend, viz. 45° S. In the narrow space between the two winds, 

 light airs, calms, and a high cross sea, with heavy rain, thunder, and lightning, gene- 

 rally prevail, and there the barometer is lowest. The belt of southerly winds lies 

 to the west of the belt of northerly winds, and the two travel laterally to the east- 

 ward, preserving their relative positions often for several days. The gale of the 13th 

 to the 20th of January, 18(Jl, as would be seen by inspecting a number of charts 

 illustrative of it, was a good example, and many others might be adduced. The 

 barometer stands higher, and the thermometer lower, in the southerly than in the 

 northerly -wind. On the western side of the former the barometer has Ijeen knowli 

 to stand as high as 30-650 inches, while in the trough, or space between the two 

 ■winds, it stood at 29000 inches. Sometimes there are several alternate belts of 

 southerly and northerly wiuds, as in gales which took place on the 27th and 28th 

 of July, 1863. 



In place of forming parallel belts, however, the two winds are often inclined, and 

 sometimes direetlj' opposed, to each other. Occasionally, too, onlj^ one of them 

 appears, the other, if it exists at all, being either above tlie surface-wind, or away 

 in the South Atlantic, to which the observations do not extend. This was the case 

 from the I4th to the 20tli of IMay, 1865, when a violent north-wester occurred in 

 the space between the meridian of Greenwich and 32° E., and the parallels of 30° 

 and 45° S. 



But whatever may be the positions of the two currents of air, the gales inva- 

 riably travel to the eastward, and many of them have been traced from the meri- 

 dian of Greenwich to 65° E. Where they originate, and how far they travel, has 

 not been determined. It does not appear that they are revolving- gales, although 

 whirlwinds may occasionally occur between the inner edges of the two wiuds ; for 

 in no instance has the wind been traced round an axis, or central area, as in the case 

 of the tropical hurricanes. They take place with so much unifonnity and regularity 

 that their progress may be traced from day to day and hour to hoiu', and the man- 

 ner of the veering or shifting of the wind, when there are two currents, be kno^vn 

 beforehand, the shift being (often suddenly) from N.E. to S.W., or from N.W.rd 

 to S.W.rd, and the veering from N.E. to North, N.W., West, &c., or with the sun. 

 They last from one to seven days, and travel at the rate of four to twenty miles an 

 hour. The wind usually sets in at N.E.rd and ends at S.W.rd, or S.E.rd. After 

 the shift, or when the wind comes to the south of west, the barometer rises, and 

 in a few hours the wind gradually abates. They succeed one another at short in- 

 tervals and with considerable regularity, but vary in force. Even the ordinary 

 changes of wind and weather in that part of the ocean seem to be more or less 

 dependent upon the antagonistic currents of air to which reference has been made. 



3. Many persons were at first little disposed to accept the ' Law of Storms ' as 

 laid down by Redfield, Ileid, Thorn, and Piddington, and there were points on 

 which these -writers themselves were, not agreed. Even at the present day there 



