Hennessy — On the Molecular Influence of Fluids. SO' 



■which the motions of siiccessive strata of the same liquid influenco 

 the adjoining strata. On this account I have not employed the 

 arrangement used by Coulomb, and subsequently applied by Meyer, 

 as the direct results Trhich that arrangement gives refer more imme- 

 diately to the resistance of a solid surface moving against the liquid. 

 E-esults as to the internal viscidity have subsequently been deduced 

 indirectly from the observations thus made. 



As the object of these experiments has been to study the pheno- 

 mena accompanying the rotation of liquids, a steady rotating motion 

 was indispensable. For this purpose I had a clock specially con- 

 structed, which worked by a gearing of toothed wheels, so as to move 

 a vertical axle carrying on it a very strong wooden disk, to which 

 another exactly similar could be fastened by screws. On the latter 

 was fixed a broad socket of hard wood, with a large hollow screw cut 

 deeply within it. The vessels employed were glass, and each carried 

 at bottom a broad solid screw of hard wood firmly attached by cement. 

 In this way the most complete union between the vessel containing 

 the liquid and the rotating support was obtained. The clock was 

 driven by two powerful springs, and was regulated by a pendulum 

 when slow motions were required. A fan with movable pallets was 

 em^^loyed as a regulator for rapid motions. The whole was supported 

 by and firmly bolted to a low flat table of wood. The circular diska 

 are furnished with a slotted arm and clamping screw which permits 

 the axis of the vessel containing liquid to be inclined at an angle to 

 the axis of rotation. The vessel which was principally employed is^ 

 of glass ; it has a bell-shaped bottom and cylindrical sides. Around 

 the cylinder a slip of paper, divided into 360 equal parts, was rolled 

 in such a way as to be in the plane of the circle forming the cross sec- 

 tion, or in a plane j)arallel to the surface of the liquid when at rest. 

 In order to observe the relative motions of the liquid and the contain- 

 ing vessel, a strong rectangular frame was fixed to firm supports^ 

 placed outside the table already mentioned, but not touching it. The 

 cross piece had a slot which permitted a small brass crotchet to be 

 screwed down vertically, after being shifted until it was placed over 

 the centre of the vessel. In this crotchet is a small slit which could 

 be closed by a screw. From the crotchet an indicator was siispended 

 by a fibre of unspun silk, such as is usually employed in galvano- 

 meters. The indicator consists of two fine slips of light wood (deal), 

 fastened with strong thread at the ends and half way from the axis. 

 At the axis a piece of very fine platinum wire was looped, and termi- 

 nated in a small hook, to which the suspending fibre of silk was 

 attached. 



The slips of deal were gi-aduated in centimetres, from the centre 

 of suspension outwardly. 



Between the two slips of deal two equal and rectangular thin 

 laminas of mica were inserted ; these laminas were always placed at 

 equal distances from the point of attachment of the suspending fibre 

 by the aid of the graduation above mentioned. Before making tlie 



