73 Mr Tregaskis 07i the Expansion of Vapour. 



Art. hi. '^Theorem for computing the Elastic Force of Fa- 

 pour. By Richard Teegaskis, Esq. in a Letter to the 

 Editor. (See Article X. in this Number.) 



Sir, Perran, near Truro, 6th Nov. 1828. 



The table in my paper on the expansion of vapour having 

 only twelve terms, requires considerable extension to render it of 

 practical utility. I have therefore to request the insertion of 

 a method for the calculation of a more extended table. 



The President of the Royal Society, who is always ready to 

 assist and encourage any attempt at further discoveries in 

 science, however humble, was requested, on his annual visit to 

 Cornwall, to look at the paper in question. He was pleased 

 with its simplicity, and, assuming my statement to be the law 

 of expansion, kindly presented me with the following theorem, 

 by which a table may be easily calculated to any subdivision 

 of parts ; or (on the assumed law) the force of vapour may 

 be found by it without a table at any distance above or below 

 the boiling point of water. — I am. Sir, very respectfully, your 

 humble servant, 



RiCHD. Tregaskis. 



To Dr Brewster. 



THEOREM. 



Let a = the number of degrees of Fahrenheit above or below 

 the boiling point. 



180 + a 



Then — =: t the temperature. 



180 



Let E = the elasticity, that of steam at the boiling point being 

 unity. 



Loff. 2 

 Then Log. E = ^^^^^ ^^ X Log. /. = 3.802 X Log. t. 



Loff. 1.2 

 Log. t = L^|— ^ X L°g- ^ = 0.263 X Log. E. 



a = 180 X < — 180= 180 xnat.no. of (^~~X Log.jE;^ — 180 

 = 180 X nat.no. of (3.802 X Log. £.) —180. 



