162 Tidal Phenomena of Hobson's Bay. 



In a limited area like Port Phillip Gulf, communicating by 

 a comparatively narrow entrance with the ocean through 

 Bass's Straits, I scarcely hoped to find such a decided result 

 as the diagram affords of the influence of atmospheric pres- 

 sure on the waters, independent of the wind currents. 



For the illustration of the question, I have selected from 

 the 30th October to the 25th December, 1863, this period 

 being inclusive of the time of the unusual and disastrous 

 floods of last year. 



The diagrams are constructed as follows : — 



The tidal range is given hourly in time, and the vertical 

 rise and fall in feet. 



The barometer and wind- readings are for the hours of 

 6 and 9 a.m., and 3 and 9 p.m., of each day. The force of 

 the wind is indicated by vertical black lines, according to 

 Glaisher's system of notation, the direction being shown by 

 red arrows. Down is a north wind, upwards is south, to 

 the right is a west, and to the left an east wind, whilst 

 intermediate points are shown by angles of corresponding 

 inclination to the chief points of the compass. 



The age of the moon is shown by the black circles under 

 the tidal diagram. 



The reading of the attached thermometer was taken, 

 but it is not used in connection with this paper, owing to 

 its bearing on the question being far too remote to be 

 entitled to notice. 



By reference to the 30th of October on the diagram, the 

 influence of a powerful north wind is evinced in counter- 

 acting the effect of a low barometer, whilst, on the other 

 hand, a west-south-west and south-south-west wind of force, 

 drives up the waters even against a rising barometer, and 

 the lessening lunar influence of the moon as she approaches 

 her first quarter. The foregoing is an exceptional contrast 

 to the usual result in showing the power of the wind, but 

 it serves as a foil subsequently, to exhibit in a stronger light 

 the overruling influence of the atmospheric column, as against 

 the atmospheric current. The moderate breezes from the 

 south-south-west and the south-west, on the 3rd November, 

 are insufficient to force up the tides under the combined 

 influence of high barometric pressure, and the low influence 

 of the moon in her first quarter ; during the three succeeding 

 days northerly and north-east winds, in some instances of 

 considerable strength, are sufficient to nullify the slight 

 fall in the barometer from the period of maximum rise on 



