Notes on Hydrology. 91 



or westerly winds. The winds from the south are cool 

 to start with; they come across the high lands of 

 Tasmania, part with a portion of their small amount of 

 moisture there ; more moisture is collected in the Straits, 

 which is condensed between Melbourne and the divid- 

 ing range about Blackwood and Macedon ; and the last 

 few drops are squeezed out about Mount Alexander and the 

 Big Hill Ranges, arriving at Sandhurst a cold dry wind, so 

 that rain is never by any chance received there with a 

 south wind. Often have I seen the sky with every 

 appearance of rain, but no matter how threatening with 

 a south wind no rain is received, excepting possibly a very 

 few drops at the commencement. Not only is there no rain, 

 but the sky becomes perfectly cloudless. The south-west 

 winds arrive laden with moisture comparatively warm over 

 the least mountainous portion of our colony, and it is from 

 that direction most of the heavy rains are received in 

 Sandhurst during the winter. 



It appears to me that in summer time the moisture is 

 brought from the sea with south-west winds ; finding the 

 land warm it is not condensed, but is carried inland over the 

 plains into Central Australia, gathering more moisture by 

 being more heated. The vapours then return with north- 

 east, north and north-west winds from a hot to a cooler 

 locality, and when deflected upwards by the first heavy 

 timber or high lands are condensed in heavy storms of short 

 duration. 



Elevation has also an influence on the amount of rainfall, 

 which may be easily understood if we bear in mind that the 

 vapour of water is lighter than air at the sea-level. This 

 vapour is invisible ; when it changes into clouds it is changed 

 partially into water. It is not known thoroughly how the 

 clouds are supported; there are several theories on the 

 subject. Nevertheless we know that clouds carrying large 

 quantities of moisture generally float several thousand feet 

 above the sea-level, and the greatest amount of rain may 

 naturally be supposed to fall at that elevation, decreasing in 

 higher and lower elevations. This has been noticed particu- 

 larly on the Ghauts in India. 



Clouds sometimes attain an altitude of upwards of 20,000 

 feet, but they cannot possibly carry much moisture. 



Respecting seasons of floods and droughts, the knowledge 

 of the general laws is at present very limited. Mr. Todd, of 

 South Australia, has pointed out that years of drought are 



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