342 History of Mechanical Inventions and 



and a piece of wire gauze is placed in the tube. This permits the exter- 

 nal air to be in regular communication with the internal air, which is fa- 

 vourable to the accuracy of the observation. See Baron Zach's Corr. Astron. 

 vol. xi. p. 391 and 480. 



3. Mr Jeffrey's Method of condensing Smoke, Metallic Vapours, &;c. 



This ingenious method, the efficacy of which is said to have been proved, 

 is represented in Plate IV. Fig. 11. The letters BB represent the flue of 

 any ordinary furnace through which the smoke rises. It is shut up at A, 

 and after turning horizontally at C, it has a descending branch D, which 

 terminates below at the opening F. The branch DD communicates at its 

 upper end with a cistern E, having its bottom perforated with holes im- 

 mediately above the flue. 



As the heated current of air, charged with smoke and vapours of differ- 

 ent kinds, descends in the branch 1)F, the constant shower of water from 

 the cistern F carries down with it the smoke and all the sublimed matter 

 from the fire, and the whole runs out through the opening F, in the state 

 of black water, without any smoke. A strong current of air is created in 

 the descending pipe by the descent of the cold water. 



The flues B and D may be close to one another, or may stand at any 

 distance, and in any relative direction. See Journal of Science, vol. xviii. 

 p. 270. 



4. Casting of Wooden Ornaments and Veneers- 

 A discovery is said to have been made in France of a method of convert- 

 ing pulverised wood or sawdust into a solid substance, by which curious 

 wooden articles may be formed in moulds, at a small expence, out of rare 

 and valuable woods. See Newton's Journal of the Arts, vol. ix. p. 35. 

 The only difficulty which is opposed to such a method consists in obtaining 

 a cement sufficiently cheap for holding together the woody particles. It is 

 evident that such a composition can never possess any of the beauty of 

 structure which is generally the principal one in rare kinds of wood, al- 

 though a coarse imitation of this may be effected by particular combina- 

 tions of different mixtures varying in colour. 



5. Account of the Lapidary's Wheel of the Hindoos- 



This wheel, used for cutting precious stones, is composed of one part 

 gum-lac, and two parts of powdered corundum (or emery.) The corun- 

 dum powder is first heated in an earthen vessel, and when the heat is such 

 as to melt the gum, it is added in portions, the whole being stirred 

 about to promote a perfect union. The paste thus made is beaten with a 

 pestle on a smooth slab of stone ; it is then rolled on a stick, and reheated 

 several times. When the mixture is uniform, it is then taken from the 

 stick and laid on the stone-table, which must be previously covered with 

 fine corundum powder, and then flattened into the shape of a wheel with 

 an iron rolling-pin : The wheel is then polished by a plate of iron and cor- 

 undum powder, and a hole is made through the middle of it by a heated 

 metallic rod. 



