130 On the Origin of Salt and Salt Lakes. 



for oxygen and hydrogen, 1 -4 for atmospheric air, and considerably 

 less for compound gases. 



I think that a table on Mr. Petrie's plan, calculated with the in- 

 dex j\ instead of ^, would prove more accurate for large alterations 

 of volume. For small alterations, the difference may not be of 

 much importance in practice, and will probably be covered by the 

 loss of heat by conduction in actual machines. 



Although I state these objections, which are precisely what I 

 should have said had I been present when Mr Petrie's papers were 

 read, I have no design to undervalue his labour and ingenuity, of 

 both of which I have a high sense. 



I may observe, that although you state in your paper that my 

 method of calculation is more difficult of application than Mr 

 Petrie's, the difference is more apparent than real ; the method of 

 calculating approximately the effect of a given compression in heat- 

 ing air is precisely the same in both, except as to the numerical 

 value of the index employed. 



To prevent the re-heating of air by friction on issuing from your 

 cooling machine, I would advise you to dispose of its mechanical 

 power by making it work a piston or turn a revolving fan, or series 

 of fans : you may thus also save some power which would otherwise 

 be lost. — Believe me, yours very sincerely, 



W. J. Macquorn Rankine. 

 Professor C. Piazzi Smyth. 



On the Origin of Salt and Salt Lakes. 



At a meeting of the Boston Society of Natural History, Pro- 

 fessor H. D. Rogers presented a communication on the origin 

 of salt and salt lakes. He thought that there was an inti- 

 mate connection between the present basins of salt water 

 and the existing distribution of the earth's climates, — a 

 connection which, fully established, promises to afford us, 

 through a tracing of the distribution of the ancient saliferous 

 deposits, much insight into the climates of the earth in the 

 past periods. A sound geological theory teaches, that the 

 original source of the salt of the ocean, and of all the salt 

 lakes, was in the chlorides of the volcanic minerals and rocks 

 of the earth's crust. The action of the descending rain is 

 id decompose these rocks, and to dissolve and float away 

 into the receptacle of the sea, the soluble salts which they 



