208 



P U M P. 



Pump. 



Descrip- 

 tion of Dr. 

 Robison's 

 improve- 

 ment on 

 Gosset and 

 Dela 

 Deuille's 

 pump with- 

 out friction. 

 TLATE 

 CCCCLXX. 

 Fig. 12. 



through the valve a;, and will therefore be forced up 

 through the valve v into the rising pipe A, because in 

 raising the cylinders the force with which they press 

 against the water presses down the mercury between ab 

 and efgh, and causes it to rise between efgh and mnop, 

 till the two mercurial columns are nearly on a level. 

 The continued rise of the cylinders causes the mer- 

 cury to fall still farther between ab and efgh, and rise 

 still higher in the space between efgh and mnop. Hence, 

 in order to balance this inequality of the columns of 

 mercury, the water rises through the valve v till the 

 height of the water in the pipe A is 13 times the dif- 

 ference of the mercurial columns. When the cylinders 

 are again depressed, more water will rise in the suction 

 pipe, and the rise of the cylinders will drive the water 

 still higher up the pipe A, and the mercury will be 

 higher in the inner than in the outer space. By con- 

 tinuing this action, the water will rise in A till the 

 mercury in the outer space arrives at the top of the 

 cylinder. Dr. Robison remarks, that with the dimen- 

 sions already mentioned, the machine will raise water 

 about 30 feet in the pipe A above x, which will make 

 the whole height above the pit-water 60 feet. The 

 machine requires to be slowly worked. 



The following are the dimensions of the three cylin- 

 ders given by Desaguliers : 



Outer Cylinder. Middle Cylinder. Inner Cylinder. 



Length, . 30 inches. 29.0 inches. 31.2 inches. 



Inner diameter, 6.74 6.35 6.03 



Thickness, . 0.10 0.08 0.13 



Outer Diameter, 6.94 6.51 6.29 



The quantity of mercury used is 36^ pounds, which 

 rises up to the height of 16 inches between the inner 

 and outer cylinder. 



With regard to the effect of this engine, Dr. Desa- 

 guliers informs us, that a man raised a hogshead of wa- 

 ter 18 feet high in a minute, but he could not continue 

 this exertion above ^th of an hour. When he wrought, 

 however, so that he could continue 6 or 8 hours a day, 

 he raised a hogshead between 10 and 11 feet in a mi- 

 nute, which Desaguliers considers as the maximum ef- 

 fect produced by a man with the best water engine. 

 Dr. Robison likewise admits that there can be no doubt 

 of the performance of this engine excelling that of any 

 other pump which raises the water to the same height. 

 He considers it as peculiarly applicable in nice experi- 

 ments for illustrating the theory of hydraulics, as it 

 would give the finest pistons for measuring the pres- 

 sures of water in pipes. 



8. Description of Dr. Robison's Improvement on Cosset 

 and De la Deuille's Pump without friction. 



Dr. Robison describes this improved pump as without 

 friction, as capable of being constructed'in a variety of 

 forms by any common carpenter, and as of great utility 

 in raising a large quantity of water to a small height, 

 or in draining marshes and marie pits, quarries, &c. 



In Fig. 12, ABCD is a square trunk, formed of four 

 planks of wood, open at both ends, and having at its 

 upper end a spout B, and a little cistern AB. Near its 

 lower end is a wooden partition, perforated with a hole, 

 in which is a clack valve E. To this wooden partition 

 is nailed a long cylindrical bag, ffff, having its up- 

 per end fixed to a round board, perforated with a hole 

 containing a valve F. This bag may be made of lea- 



ther, or of double canvass, a fold of thin leather, or of 

 sheep's skin, being placed between the two folds. The 

 upper end of the bag should be firmly tied with a cord, 

 in a groove turned out of the rim of the board at F. 

 Into the board at F is fixed the fork of the piston rod 

 FG, and the bag is kept distended by a number of 

 wooden hoops or rings of strong wire ff, ff, and fix- 

 ed to it at a few inches distance from one another, and 

 kept at the same distance by three or four cords, bind- 

 ing them together, and stretching from the top to the 

 bottom of the bag. The distance of the hoops should 

 be nearly twice the breadth of the rim of the wooden 

 ring to which the upper valve F, and piston rod FG, 

 are attached. 



f Now let this trunk," says Dr. Robison, " be im- 

 mersed in the water. It is evident that if the bag be 

 stretched from the compressed form which its own 

 weight will give it, by drawing up the piston rod, its 

 capacity will be enlarged, the valve F will be shut by 

 its own weight, the air in the bag will be rarefied, and 

 the atmosphere will press the water into the bag. 

 When the rod is thrust down again, the water will come 

 out by the valve F, and fill part of the trunk. A re- 

 petition of the operation will have a similar effect ; the 

 trunk will be filled, and the water will at last be dis- 

 charged by the spout. 



" Here is a pump without friction, and perfectly 

 tight ; for the leather between the folds of canvass ren- 

 ders the bag impervious both to air and water. We 

 know from experiment, that a bag of 6 inches diame- 

 ter, made of sail cloth No. 3, with a sheep skin between, 

 will bear a column of 15 feet of water, and stand 6 

 hours work per day for a month, without failure, and 

 that the pump is considerably superior in effect to a 

 common pump of the same dimensions. We must only 

 observe, that the length of the bag must be three times 

 the intended length of the stroke ; so that, when the 

 piston rod is in its highest position, the angles or ridges 

 of the bag may be pretty acute. If the bag be more 

 stretched than this, the force which must be exerted 

 by the labourer becomes much greater than the weight 

 of the column of water which he is raising. If the 

 pump be laid aslope in these occasional and hasty 

 drawings, it is necessary to make a guide for the piston 

 rod within the trunk, that the bag may play up and 

 down without rubbing on the sides, which would quick- 

 ly wear it out. 



" The experienced reader will see, that this pump is 

 very like that of Gosset and De la Deuille, described 

 by Belidor, vol. ii. p. 130, and most writers on hy- 

 draulics. It would be still more like it, if the bag were 

 on the under side of the partition E, and placed farther 

 down the pump ; but we think that our form is greatly 

 preferable in point of strength. When in the other si- 

 tuation, the column of water lifted by the piston tends 

 to burst the bag, and that with a great force, as the in- 

 telligent reader well knows. But, in the form recom- 

 mended here, the bag is compressed, and the strain on 

 each part may be made much less than that which tends 

 to burst a bag of 6 inches diameter. The nearer the 

 rings are placed to each other, the smaller will the 

 strain be. 



" The same bag piston may be employed for a forcing 

 pump, by placing it below the partition, and inverting 

 the valve; and it will then be equally strong, because 

 the resistance in this case too will act by compression." * 

 A double pump, of a nature similar to that which has 



Pump. 



Robison's System of MecJtamcal Philosophy, vol. ii. p. 677. 



