148 History qf Mechanical Inventions 



consisting of a hollow bulb and tube of platina containing air, the ex- 

 pansions of which were indicated by its action upon a column of mer- 

 cury in a vertical glass tube, contained within the platinum tube. Dr. 

 Ure, however, in his Dictionary of Chemistry, published in 1821, has 

 proposed the very same instrument, with more minute details respecting 

 the -graduation of its scale. These two instruments are fully described 

 in the article Pyrometer, in the Edinburgh Encyclopedia, Vol. XVII. 

 Part I. just published. 



Art. XXVII. HISTORY OF MECHANICAL INVENTIONS AND 

 PROCESSES IN THE USEFUL ARTS. 



1. Mr. Yallanee , s Apparatus for Freezing Water. 



The apparatus described by Mr. Vallance is represented in Plate II. 

 Fig. 17. and is founded on the principles of freezing in the air- pump, 

 discovered, as our author states, by Dr. Cullen and Mr. Nairne, and im- 

 proved by Mr. Leslie. This method consists in passing a current of di y 

 rarefied air over the extended surface of the water, which, by impinging 

 on it, carries ofF the aqueous vapours. The water is placed in a flat bot- 

 tomed vessel aa, so as to form a stratum about half an inch deep. The 

 greater part of the air is to be removed from the vessel through the pipe 

 b by two good air-pumps, till the pressure of the air within will support 

 about an inch of mercury. A hollow tube c passes through a stuffing- 

 box in the lid of the vessel a, where it is enabled to slide up and down, 

 and has at its bottom a circular plate or disc d, which is brought to 

 within half an inch of the water, and rises a little conically in the mid- 

 dle. 



Another similar tube e is attached by flanges to the upper end of c, 

 and passes through a stuffing-box in the upper vessel f, from which a 

 bent pipe g passes into another vessel /*, which is nearly filled with leaden 

 bullets, and has a small air-hole below. A quantity of sulphuric acid is 

 occasionally poured on these bullets to wet their Eiirface. Several air- 

 holes are made in the lid of the vessel a, and also in the plate d, and are 

 closed air-tight with glass, to show the progress of the operation within. 



When the air-pump draws the air from the vessel a, the stop-cock g is 

 to be partly opened, so as to admit air as fast as the pumps exhaust it. 

 This air passes between the leaden balls in h, and has its aqueous 

 particles absorbed by the acid. A current of dry air thus passes from h 

 through the pipe g, the vessel^ the tubes e and c, and descends upon the 

 surface of the water under the disc d in the vessel a. This current of air 

 being drawn off by the air-pumps through b, carries off the heat from 

 the water and makes it freeze. After one stratum is frozen, more water 

 is introduced into a, so as to be about half an inch deep on the ice, and 

 this is frozen by a similar operation, till the vessel is filled up with ice. 

 See Newton's Journal of the Arts, vol. viii. p. 251. 



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