MECHANICAL AND USEFUL ARTS. 27 



are still in a fair way to banish more than a third of, at least, 

 , S00,000 farm-horses in Great Britain. With the saved produce of a 

 million and a half acres (which these hungry teams now consume), 

 added to the eight bushels additional yield on 4,000,000 acres of 

 wheat — to say nothing of the augmented production of vegetable 

 food and butchers' meat, from the yet unforeseen revolution in clay- 

 land management, and on other soils from the interpolation of more 

 than a single crop in one year, and the entire remodelling of our 

 system of rotations, — we may be able before long to feed our popu- 

 lation, independent of American prairies, and no longer import sup- 

 plies equal to a fourth of our harvest." 



NEW HYDRAULIC CEMENT. 



Captain Hunt has read to the American Association a paper on 

 Hydraulic Cement, prepared by Lieut. Q. A. Gilmore. His deduc- 

 tions from a long series of observations are : — 1. In cold weather, 

 when it is necessary that the cement should harden quickly, warm 

 water should be used for mixing the mortar and wetting the solid mate- 

 rials with which it is to be used. In warm weather, on the contrary, 

 cool water should be used for the same purpose, in order to delay 

 the setting until the mortar is laid in position. 2. The time re- 

 quired by the cement to set (if within the ordinary limits of 1 "10 of 

 an hour to 1^ hour) furnishes no means of judging of the ultimate 

 strength and hardness which it is likely to attain. 3. It is not 

 probable that while the present method of manufacturing cement is 

 pursued in this countiy, we can produce an article equal to Parker's 

 Eoinaa cement, or the artificial Portland cement from abroad. 

 4. The stone furnishing what is generally termed intermediate 

 lime, now rejected by our manufacturers as worthless, on account of 

 its containing an excess of caustic lime, may be used with entire 

 safety if combined with five or eight per cent, of an alkaline silicate : 

 " soluble glass" is a good silicate for that purpose. 5. The maxi- 

 mum adhesion to stone is secured by mixing the cement paste,* or 

 mortar, very thin (en coulis) rather than very stiff. The maximum 

 density, cohesion, and hardness, on the contrary, are all incom- 

 patible with this condition. 0. Cement should be ground to an im- 

 palpable powder, when it is intended to give mortar its full dose of 

 sand, the coarse particles of sand being a poor substitute for that 

 article. Finally, all the stone which does not effloresce with dilute 

 hydrochloric acid, or which, during calcination, has been carried 

 beyond the point of complete expulsion of carbonic acid ga^', should 

 be rejected. 



NEW PORTABLE COFFER-DAM. 



Captain Hunt has also read to the American Association a de- 

 scription of a New Portable Coffer-dam, the idea 'of which occurred 

 to him and was put in practice while he was superintending certain 

 constructions at Fort Taylor, Key West. The novel feature of this 

 coffer-dam is this : — Make a strong canvas case for the whole 

 coffer, using two thicknesses of canvas, and interposing a complete 



