106 



GENERAL OBSERVATIONS ON THE W:NDS. 



There is no question that the figures in this Table may be open to some 

 doubt, as the subject is a difficult one, and they are given independently 

 of the different forces exerted by aqueous vapour and by air. Sir Henry 

 James also drew up a more complicated Table, and foreign and other 

 observers have also prepared various other Tables, differing more or less 

 from that just given, which will suffice for the sailor's use. 



The results vary much, according to the form of Anemometer used. 

 From exhaustive experiments recently made by Mr. W. H. Dines, it was 

 found that the pressure upon a plane area of fairly compact form is about 

 li lb. per square foot at a velocity of 21 miles per hour, or in other words, 

 a pressure of 1 lb. per square foot is caused by a wind of a little more 

 than 17 miles per hour.* 



After the disastrous failure of the Tay Railway Bridge and other struc- 

 tures, owing to heavy gales, an inquiry into the subject led Mr. Thomas 

 Hawksley to draw up the following Table.f 



(18.) The Alternation of tkt Sea and Land Breezes in warm latitudes is 

 an important feature in coast navigation. Its cause is generally well under- 

 stood, being owing to the different powers of radiation and absorption of 

 heat possessed by land and water. So that, generally, when the day tem- 

 perature is highest on the land, the strongest will be the alternating 

 breezes. During the day the radiation of the sun's heat on the land 

 causes the air to expand and rise from the surface, and then the sea air 

 rushes in to fill the void. It frequently occurs that the surface of the soil 

 will show a temperature of 120° under the meridian sun, and sink to 50° 

 or 60° during' the night ; while the sea, rarely having a higher temperature 

 than 80°, and from being a bad radiator fluctuates but very Uttle, it follows 

 that it is alternately warmer and colder than the land, and hence the phe- 

 nomena in question. The minimum temperature of the twenty-four hours 

 occurring a little before sunrise, and the maximum about 2 p.m., the change 

 of these breezes occurs generally at some little time after those hours. 



The above theory of Land and Sea Breezes does not satisfactorily account 

 for the known fact that the Sea Breeze always sets in first in the offing, 



* An account of these experiments is given in the Report of the Meteorological 

 Council to the Royal Society, for the year ending March 31st, 1890, pp. 36 — 45. 



t "On the Pressure of the Wind upon a Fixed Phxne Surface," by Thos. Hawksley, 

 C.E., F.R.S., in the Report of the British Association, 1881, pp. 480—482. 



