Tidal Phenomena of Hobson's Bay. 161 



Art. XXXIX. — Notes on the Tidal Phenomena of Hobson's 

 Bay, as affecting the discharge of Flood Wcders from the 

 River Yarra. By Thomas E. Rawlinson. C.E. 



[Read Oct. 31st, 1864.] 



The importance of the River Yarra to the City of Mel- 

 bourne, taken in connection with the large and yearly 

 increasing value of the interests which have arisen on its 

 banks, and the prospective value of the low lands liable to 

 flood between Melbourne and the Bay, renders the question 

 of dealing with the flood waters to which it is subject one 

 of no ordinary importance. 



The late frequent and disastrous floods, have further given 

 the subject an immediately practical character. 



Various plans have been proposed from time to time for 

 relieving the river in time of Hood, nearly all of which 

 (indeed all the plans of a practical character) contemplate 

 the locality of Hobson's Bay as the only available site for 

 the discharge of the surplus waters, and it is in reference to 

 this part of the- question that I propose to offer a few 

 remarks in connection with the tidal diagrams submitted. 

 The Bay being the only available outlet for drainage pur- 

 poses, any influence acting upon its waters, tending to 

 elevate or depress their general level, must as a consequence 

 have a reflex influence upon the drainage, by diminishing or 

 increasing the available fall for the waters, and propor- 

 tionately reduce or enlarge the discharging power of the 

 water courses, and it is proposed in this paper to illustrate 

 the tidal phenomena in the Bay, having such tendency to 

 diminish or increase the drainage power of the river. 



The fact of unusual high tides in Hobson's Bay, and their 

 retention at an unusual height for lengthened periods, is 

 not unknown, and from their being usually accompanied by 

 south-westerly winds, the high tides are attributed to such 

 winds. 



The object of this paper and accompanying diagrams is 

 to show that the winds are but a secondary cause, and may 

 perhaps be more correctly termed a consequence or result of 

 the prime cause, namely, atmospheric disturbance, of which 

 the barometer is the index ; the heaping up of the waters, 

 or their depression, independent of the diurnal tides, being 

 but the reflex of the atmospheric wave, acting upon the 

 surface of the waters. 



M 



