164 Tidal Phenomena of Hobson's Bay. 



atmospheric pressure and -wind currents from the south- 

 west, a continuous heaping up of the waters of the Bay for 

 a period of forty-eight hours, nearly three feet above their 

 ordinary level. Of this result, about two feet is due to the 

 atmospheric pressure alone, the rest being assumed to be 

 due to the heavy gale blowing from the south-west. 



The continuance of gales of wind from the west-south- 

 west and south-west, ending iu the moderate breezes of the 

 18th from the same quarters, to some extent checked the 

 beneficial influence of a rising barometer, until, on the 20th 

 of December, the maximum rise again resulted in a maximum 

 fall of the water level. 



From the 20th to the 25th of December, a slight fall in 

 the barometer, accompanied by breezes from the south and 

 south-west, slightly elevates the waters as before. 



The singular oscillation on the face of the tidal wave on 

 the night of the 2.3rd of December,* appears to be owing to 

 some inequality of force and direction of the north-east 

 gale, before and' after 9 pin. This inequality I think 

 probable because of the wind having at 3 p.m. been blowing 

 stiffly from the opposite quarter, to which point it had again 

 veered at 6 a.m. on the 24th, from which point, with slight 

 variation, it continued to blow during the remainder of the 

 24th of December. The slight fall of the barometer may also 

 have had an influence between the gusts of wind. 



From the facts narrated in this paper and shown on the 

 diagram, it will be readily understood that the capacity of 

 the river Yarra for discharging its surplus waters is very 

 materially affected by the atmospheric pressure and winds 

 acting upon the waters of the Bay, and that, in the absence 

 of a sufficient fall, the low lying lands around the city and 

 mouth of the Yarra cannot be effectually drained by 

 ordinary channels &i> all times. 



Under ordinary circumstances the mean tide level of the 

 Yarra, opposite to the Custom House, is nearly twenty-six 

 inches higher than the mean tide level at Williamstown. 

 This difference gives about 3f inches or "31 of a foot fall per 

 mile. 



On the 14th and 15th of December, the waters of the Bay 

 were continuously heaped up for a period of forty-eight 

 hours to nearly three feet above the ordinary level of the 

 tides, and the ebb-tide was retained to a yet greater difference 



* Note.— Not shewn in reduced diagram. 



