IN RAISING BURTHENS. 411 



$ular and horizontal indentations are cut out quite around it, and 

 at proper distances, according to the thickness to be given to the 

 millstones. Wedges of willow, dried in an oven, are then driven 

 into the indentations, by means of a mallet. When the wedges 

 liave sunk to a proper depth, they are moistened, or exposed to the 

 humidity of the night, and next morning the different pieces are 

 found separated from each other. Such is the process which, ac- 

 cording to M. de Mairan, is employed in different places for mak- 

 ing millstones. 



By what mechanism is this effect produced 1 This question hafc 

 been proposed by M. de Mairan 5 but in our opinion, the answer 

 which he gives to it is very unsatisfactory. It appears to us to be 

 the effect of the attraction by which the water is made to rise in the 

 exceedingly narrow capillary tubes with which the wood is filled 

 Let us suppose the diameter of one of these tubes to be only the 

 hundredth part of a line ; let us suppose also, that the inclination 

 of the sides is one second, and that the force with which the water 

 tends to introduce itself into the tube, is the fourth part of a grain : 

 this force, so very small, will tend to separate the flexible sides to 

 the tube, with a force of about 50,000 grains ; which make about 

 8j pounds. Tn the length of an inch let there be only 50 of these 

 tubes, which gives 2500 in a square inch, and the result will be an 

 effort of 21875 pounds. As the head of a wedge, of the kind 

 abovementioned, may contain four or five square inches, the force 

 it exerts will be equal to about QQ or 100 thousand pounds; and if 

 we suppose 10 of these wedges in the whole circumference of the 

 cylinder, intended to form millstones, they will exercise together an 

 effort of 900 thousand or a million of pounds. It needs, there- 

 fore, excite no surprise that they should separate those blocks into 

 the intervals between which they are introduced. 



[Ilutlon. Montucla's Ozanam, 



END 01 VOL. III. 



li, Wilks, Printer, 89, Chancery-lane, London. 







