The device used to facilitate the addition  of drilling  fluid additives  to the    whole    mud system.    While several types of hoppers exist, they generally have  a  high  velocity stream  of  mud  going  through  them  and  a means of mixing either dry or liquid mud additives into the whole mud stream. The resultant mixed mud is then circulated back into the surface mud system. A hopper is generally used to introduce relatively small quantities of additives to the mud system.