VIS A. TERGO. 15 



The indication of tlie existence of what is called a vis a tergo, and 

 the illustration of its power usually given, is one supplied by an experi- 

 ment devised by Hales to measure, as he thought, the force with 

 which sap ascends in a stem. A tube, bent so as to ascend, descend, 

 and ascend again, had a quantity of mercury introduced into the 

 curvature between the descending and re-ascending portion, and was 

 attached firmly to a stem, the top of which had been cut off. It was 

 fastened on the stem by means of a copper cap which was secured by 

 a lute, and this was covered with a piece of bladder firmly bound 

 above and below the upper and the lower extremities of the cap ; the 

 level of the mercury at the commencement of the experiment was 

 noted, and as the sap rose in the stem, and through the stem into 

 the ascending limb of the tube, which it did, the pressure caused the 

 mercury to sink in the descending limb and rise in the ascending 

 one ; and thus could the pressui*e occasioned by the ascending sap 

 be measured. 



Brucke, experimenting with such an apparatus, found that in a 

 vine the spring sap, having a specific gravity of TOOOS, raised a 

 column of mercury to the height of 4| inches, and therefore must 

 have created a pressure equal to that of a column of water 195 inches 

 high. In another experiment, sap of specific gravity 1-0009 raised 

 mercury to the height of 17| inches. 



The force of the pressure in one experiment which was tried was 

 found to be equal to 38 inches of mercury, which Hales states is 

 nearly five times greater than the force of the blood in the crural 

 artery of a horse, and seven times greater than the force of the blood 

 in the same artery of a dog. 



These statements I accept unhesitatingly. I know not what 

 precautions were adopted against error, and I cannot divest my mind 

 of the impression that the force of the pressure may have been due in 

 some measure to the elasticity of the stem released of the pressure of 

 the portion of the tree cut off. Knowing of no vis a tergo but that of 

 endosmose, I accept the facts as an approximate measurement of that 

 action, and an indication of the rapidity with which it is carried on ; 

 but I do so only provisionally. The impression to which I have 

 referred remains. I have oftener than once cut down a banana plant 

 in vigorous growth, and in a few minutes seen the inner portion half 

 an inch or more above and beyond tlie entire integument, which may 

 have been occasioned by rapid growth ; but in the one case as in the 

 other, in the absence of certain knowledge, I cannot divest myself of 

 the impression that the phenomena may have been to some extent 



