Mechanical Science. 221 



for the French buhr, especially in the grinding' of wheat, for they 

 alone possessed the required degrees of hardness, toughness, 

 and that cellular structure which is necessary to perform the 

 operation rapidly, without causing the mixture of earlhy matter 

 with the flour, or without heating it. 



The French buhr occurs in detached masses, or blocks, in 

 the fresh-water limestone formation of the Seine. It is inter- 

 mediate between hornstone and calcedony. No similar stone 

 has been found in the analogous formation of the Isle of Wight, 

 or any where else in this country, but the stone which Mr. 

 Bishop has successfully substituted for it occurs near the 

 middle of the eastern ridge of the Halkin mountain, which itself 

 is composed of limestone rocks. The buhrstone, or entrochital 

 hornstone, appears as a bed about four yards thick, dipping 

 easterly, like all the other strata of the mountain. It some- 

 times varies in quality, rotten masses occurring, and also por- 

 tions that are too close for the miller. Still the corallite struc- 

 ture pervades the whole, the entrochites being perfect and 

 entire in some instances, while in the chief part of the bed the 

 casts alone remain, thus leaving the rock vesicular ; and in this 

 respect differing from the nature of the pores in the French 

 buhr, which appear to have been caused by erosion, their edges 

 being rusty and impure, whereas those in the Halkin buhrs are 

 of pure flint, and exceedmgly sharp and hard. 



These stones have now been tried for the last three years in 

 many mills, and evidence of their excellence and superiority in 

 all respects, of the most satifactory kind, has been obtained. 

 Hence there is no doubt they will get into general use, and the 

 more they are made known the better. — Trans. Soc. Arts, xxxix. 

 55. 



6. Oil for Watch-work, &c.— Oil used for diminishing fric- 

 tion in delicate machinery, should be free from all acid and 

 mucilage. The following is the process : (M. Chevreul's) re- 

 commended as the most convenient for procuring it in the most 

 favourable state. Put into a matrass or glass flask a portion 

 of any fine oil, with seven or eight times its weight of alcohol, 

 and heat the mixture almost to boiling, decant the clear upper 

 stratum of fluid and suffer it to cool ; a solid portion of fatty 

 matter separates which is to be removed, and then the alcoholic 

 solution evaporated in a retort or basin until reduced to one- 

 fifth its bulk. The elaine or fluid part of the oil will be depo- 

 sited. It should be colourless and tasteless, almost free from 

 smell, without action on infusion of litmus, having the con- 

 sistence of white olive-oil, and not easily congealable. 



7. On the setting of cutting Instruments. — Mr. Reveley re- 

 commends the use of soap instead of oil for the setting of 



