22 DAIRY ANALYSIS 



when the speed slackens, a few further pulls are neces- 

 sary. If a string with two handles is provided, this is 

 wound once completely round the spindle (or the 

 pulley on the spindle) ; take a handle in each hand, 

 and pull with the right hand, keeping the string taut 

 with the left hand, and at the end of the pull continue 

 the motion of the left hand to loosen the string round 

 the spindle ; pull back with the left hand keeping the 

 string very loose, and then repeat the stroke. This 

 method of driving requires care, but is, when learnt, a 

 most satisfactory way of spinning the disc ; when a 

 sufficient speed is attained the string is allowed to 

 hang loosely, the handles being placed on the bench 

 at each side. If the string is not properly loosened 

 at the end of the stroke, or on the return stroke, it 

 winds up, and the handles must then be immediately 

 dropped, or the hands may receive a nasty blow. 

 Unbleached blind-cord is a suitable material for the 

 string, and a good supply should be kept, as the string 

 wears. 



During the running the disc may be kept warm by 

 means of a Bunsen burner or a spirit-lamp placed 

 underneath near the edge, with a flame so adjusted 

 that it just touches the disc. 



The disc should be stopped gently, not suddenly, 

 and the cover unscrewed. Do not take hold of the 

 boss, in the centre of the cover, when the machine is 

 running at high speed. 



If the disc is not kept warm, place the bottles, after 

 stopping the machine and removing the cover, in water 

 kept at 60 to 70 C. ; the small tank provided with 

 the apparatus is nearly filled with water at the required 

 temperature, and a spirit-lamp or Bunsen burner used 

 to keep it warm. 



After one minute the bottles may be read ; hold the 

 bottle (Fig. 16) by the top with the left hand and by 

 the cork with the right, at a level with the eye ; the 

 position of the bottle should be as nearly vertical as 

 possible. By gently moving the cork, the lower level 



