H R 



Expantiorfpf water. ( UreJ 



The maximum density of water is at 39., and it is a singular fact that 

 the expansion of water is the same for any number of degrees above or 

 below the maximum of density ; thus the density of water at 32 and at 

 4fio is precisely the same. The following Table, the result of experi- 

 ments by Sir Charles Blagden and Mr Gilpin, shews this in a clear light. 



This law of maximum density does not prevail in the case of sea water ; 

 on the contrary, Dr Marcet found that sea water gradually increases in 

 weight down to the freezing point. 



HORIZON, Dip or depression of. 



In observing an altitude at sea with the sextant or reflecting circle, 

 the image of the object is made to coincide with the visible horizon, but 

 as the eye is elevated above the surface of the sea by the height of the 

 ship's deck, the visible horizon will be below the true horizontal plane. 



The following Table gives the dip or apparent depression of the hori- 

 zon for different elevations of the eye, allowing y-r for terrestrial refrac- 

 tion. The dip must be always subtracted from the observed altitude 

 when taken, by the fore obscrvatioDj but added to it in the back observa- 

 tion. 



145 



