TEEATMENT OF THE COMSTOOK OEES. 223 



nished with shoes, between which, when attached to the muller, there is a 

 channel or radial passage left for the circulation of the pulp. The muller also 

 contains radial grooves between the shoes, so that, when the latter wear 

 down, the channel may still be large enough to permit an easy movement of 

 the material. The muller is raised or lowered by means of a screw and 

 movable nut at the top of the hub, the screw resting on the top of the driv- 

 ing shaft, to which the hub is keyed. The circulation of the pulp in this 

 pan is eiFected without the use of wings or guides, such as are commonly 

 employed in other pans for this purpose. When the muller is in motion the 

 pulp, passing between the grinding surfaces, from the center to the circum- 

 ference of the pan, descends again by its own weight toward the center, on 

 the upper side of the muller ; a movement promoted by the conical shape of 

 the muller-plate. In the use of guide-plates or wings to aid the circulation 

 there is sometimes a difficulty experienced in the tendency of coarse sand to 

 settle and pack firmly, if the pan is stopped for a little while, and giving 

 much trouble in starting again. By thus dispensing with the use of wings 

 some inconvenience is avoided. The charge of the pan is about 1,500 

 pounds, usually working four hours on a charge. It runs at 60 or 70 revo- 

 lutions per minute. 



"Wheeler and Randall's Pan. — Fig. 1, on Plate XXI, presents a view 

 of _a pan known as the Excelsior, devised' by Wheeler and Randall. This 

 pan differs from those before described chiefly in the form of the bottom, 

 which is conoidal. The object of this device is to produce surfaces of such 

 form as to insure perfect uniformity of wear and the highest degree of grind- 

 ing effect. Its efficiency, in this respect, is attested by the experience of 

 practical mill-men. It is not, however, so generally used as the ordinary 

 Wheeler or other pans already mentioned. 



The dies, muller, and shoes have, of course, a form corresponding to 

 that of the pan-bottom. They are secured in place in much the same way as 

 in the Wheeler pan. There are guide-plates to assist in directing the move- 

 ment of the pulp, and there are openings in the muller between the shoes for 

 its free passage between the grinding surfaces. The gearing of the pan, step- 

 box, and driving shaft, and means of raising the muller, do not differ materi- 

 ally from the common Wheeler pan. This pan is made of various sizes ; the 



