Mr. Walker -oh the Artificial Production of Cold. 1 1 



sents with so much accuracy distances on the meridian, is no 

 less exact when applied to measurements perpendicular to the 

 meridian. At least this is the case in England in the instance 

 we have examined ; and a little time will show whether the 

 same conclusion is confirmed or contradicted by the geode- 

 tical operations now executing on the continent. 



June 13, 1828. J. IvORY. 



IV. Additional Remarks on the Artificial Production of Cold. 

 By Richard Walker, Esq., of Oxford. 



To the Editors of the Philosophical Magazine and Annals. 

 Gentlemen, 

 PERCEIVING an erratum in my communication inserted 

 ■*■ in the Philosophical Magazine for the present month, 

 which renders my meaning rather obscure, I request you will 

 have the goodness, in your next, to correct it thus : — at page 

 404?, 8th line from the bottom^r " freely" — read "freshly." 



I shall avail myself of this opportunity to present, in a cursory 

 way, a few additional observations. 



The drier and finer the mixed powder of sal ammoniac 

 and nitre is, the better ; and the pulverization is best effected, 

 in the first instance, with a heated pestle. Glauber's salt in 

 an efflorescent state, or which by long keeping or from ac- 

 cess of air has changed to a powder, is unfit for the purpose ; 

 in this state producing heat by solution in water. The best 

 way of preparing the frigorific mixture is by previously pla- 

 cing the powdered Glauber's salt, and giving it a level surface, 

 at the bottom of the vessel, and upon that the mixed powder 

 of sal ammoniac and nitre ; adding first about half the quan- 

 tity of water, and immediately after the remaining portion, 

 stirring the whole together each time. The vessel containing 

 the powdered salts, as above stated, may remain thus any con- 

 venient time before adding the water. [Care must be taken 

 to stir the evaporating mixture towards the end of the process, 

 and not to urge it too far.] Nitre being a much cheaper ar- 

 ticle than sal ammoniac, more easily reduced to powder, and 

 producing about 16 degrees of cold by solution in water, may 

 supersede the use of the mixed powder for cooling the watef 

 in which wine is placed. This powder, moreover, is useful, 

 occasionally, as an addition to mixtures of ice and salt, to in- 

 crease the power and accelerate the process. 



The proportions of the articles given in my former paper 

 are adapted to the temperature of 50° ; at a higher tempera- 



2 C ture, 



