22 THE VOYAGE OF H.M.S. CHALLENGER. 



" 1. Screw in, at E, the manometer tube, and at H the regulating pump. 



" 2. Pour in the layer of mercury, and on it that of castor oil. Fill the pump 

 with glycerine, and insert its piston, taking care to exclude air-bubbles. 



" 3. Insert the gun-metal part K. Its bearing (at s) on the rim of the cast-iron 

 base-piece must not be made with leather, but with a ring of india-rubber, or of very 

 uniform cardboard. The fixing down of this part, by means of the (six) screws, must 

 be done with great exactness : — otherwise (thick as it is) it might suffer a very slight 

 distortion, and the piston PP would not work in it. 



" 4. After pouring in, if necessary, some more castor oil, insert very cautiously 

 the piston PP, carefully wiped, and then anointed with castor oil. To put it in, it is 

 to be held by means of A, which, for this purpose, is screwed into the middle of it. 

 During the insertion of the piston the hole b is left open to allow of the escape of air 

 and (possible) excess of castor oil. Close 6 by means of its screw, the piston being 

 held at the desired height. Take out A, and screw B into the piston in place of it. 



"5. Put on the part MM — after inserting in it the small piston pp, with its 

 cylinder nn — in such a way that the rod cc may pass between the two studs d on the 

 piston PP, opposite to the opening FF. 



" 6. Pour a little treacle over the small piston at aa ; screw on the piece NN, and 

 fill it with glycerine ; then adjust to NN the coupling -tube of the compression 

 apparatus, which should be filled with glycerine or with glycerine and water. 



" Observations. 



" It is not necessary that the whole space between the mercury and the piston PP 

 should be filled with castor oil. A layer of glycerine and water may be placed over 

 the mercury, then a thin layer of the oil. In fact, the regulating pump is full of 

 glycerine and water. 



" The rod cc is placed as shown to give a simultaneous rotation to the two pistons, 

 so as to overcome stiction. 



" It should be moved slowly, and in such a way as to exert no vertical force upon 

 the piston PP. It ought to be pushed by a vertical straight-edge, moved hori- 

 zontally. One can judge of the delicacy of the apparatus by the displacement of the 

 mercury column when the slightest vertical pressure is exerted on the rod. 



" I will again call attention to the scrupulous care which must be bestowed on the 

 pistons and on the cylinders in which they work : — the slightest scratch, due to dust, 

 would make it necessary to retouch these surfaces ; and after several retouchings they 

 will become too loose. 



" The manometer tube, which is to be cemented into the iron piece which screws 

 into E, should be chosen of small enough diameter to prevent sensible change of level 



