330 Salt Rain 
cistern (No. 3.) ought not to overrate the 
quantity of salt, because it was mixed with 
less or more of other water previously in the 
cistern ; but then as a compensation for this 
mixture there is the quantity of muriatic salt 
arising from the washing of the slates, if any 
such exist. This quantity (supposing it to 
exist) may be considered as constant, and 
may be inferred from the result stated in No. 
4; and it is a curious coincidence that the 
slates of the house appear to furnish the same 
quantity of salt to the cistern water as the 
rain-gage bottle does to its contents. If 
therefore we take .18 from .81 we shall have 
.68 remaining, for the quantity of salt, which 
must be Jess than that brought by the rain, 
by reason of the mixture above mentioned. 
Now as No. 2 (.86) must be more than the 
correct number, and this last (.68) must be 
less than the correct number, the mean (.77) 
may be safely inferred as a near approxima- 
tion to the true quantity of salt in 7500 grains 
of rain water which fell on the Sth instant. 
In round numbers 1 grain of salt in 10000 
erains of water. But sea water contains 1 
grain of salt in 25 water; hence 1 grain of 
sea water must have been found in every 400 
grains of rain water, on that memorable oc- 
casion. 
