TRANSACTIONS OF THE SECTIONS. 157 



release, and compression are related to each other in such a manner that, in 

 designing a slide-valve motion, the fixing of any three of those points for a given 

 position of the link fixes the fourth point also. For example, suppose that in a 

 certain position of the link, the positions of the eccentrics and the lap or cover at 

 the eduction-edge of the cylinder-port are so adjusted as to give a certain rate of 

 expansion: then the only element remaining capable of adjustment is the cover 

 at the eduction-edge of the port ; and that element, when it is fixed, fixes at once 

 the release and the compression ; and it often happens that the best positions of 

 the points of release and of compression are inconsistent with each other ; so that 

 a compromise has to be made. That objection, in some examples of slide-valve 

 motions, has been overcome by the use of double-slides; but in all the double 

 slide-valve motions hitherto introduced, there exists the defect of complexity in 

 construction and working; for in addition to the ordinary handle of the link- 

 motion, a second handle has to be used in varjang the rate of expansion. The 

 authors of this paper propose to accomplish the same residt in a very simple way, 

 by giving a small sliding motion to that part of the valve-seat which contains the 

 induction-edges of the cylinder-ports, so as alternately to contract and enlarge 

 those ports at each stroke of the engine. The only mechanism required, in addi- 

 tion to the ordinary slide-valve gear, consists in the moveable seat, with a rod 

 and a third eccentric to give it motion : the rate of expansion is varied, when 

 required, by shifting the link in the ordinary way by the use of the ordinary handle 

 alone ; yet the effect is the same as if the admission and the exhaust of the steam 

 were regulated by two different slide-valves, each with its own link-motion and 

 pau" of eccentrics. Hence, in designing the valve-motion, the points of release and 

 compression can be adjusted to the best positions, independentlj^ of the points of 

 admission and cut-off. The authors consider that the moveable seat which they 

 propose ought to be used together with a kind of slide-valve on which the pres- 

 sure of the steam is balanced, such as that introduced by Mr. Thomas xA.dams, in 

 order that the different rates of travel of the slide-valve over the fixed and 

 moveable parts of the valve-seat may not produce unequal wear. 



On the Consumption of Fuel. By William Patesson. 



On some of the Difficulties the Scientific Engineer meets with in Practice. 



By W. W. Uequhaet. 



APPENDIX. 



The Relation of the Upper and Lower Crags in Norfolk. 

 By JoHJf E. Taylor, Hon. Sec. Norwich Oeol. Soc. 



The object of this paper was to prove that the present classification of shells in 

 the Norwich Crag is ^imperfect on account of an upper bed being included in the 

 Crag. The mean percentage of the shells from the two crags makes the relation 

 of the Red and Norwich Crags very dissimilar, whereas there is really a near con- 

 nexion between them. By separating the shells of the upper bed, the underlying 

 Norwich Crag approaches the Red, whilst the upper bed itself forms a graduating 

 link between the three Crags and the overlying Drift beds. 



After giving the established percentages of recent and extinct shells in the three 

 Crags, as well as the proportion of arctic shells foimd in them, the author men- 

 tioned several places in ^lOrfolk where the Upper Crag may be seen overlying the 

 Norwich Crag, as at Coltishall, Horstead, Trowse, Thorpe, Whitlingham, and 

 Bramerton. The height of the upper bed ranges above the lower from 3 to 15 feet. 

 It is marked by the total absence of freshwater shells, by the paucity of littoral 

 species, and by the abundance of deeper sea-shells. It is also distinguished by 

 the greater abundance of arctic species, as at Bramerton and Thorpe, where several 

 species of Astarte, Cyprina ishtncUca^ Cardium groenlandicum, Lucina borealis, and 

 others abound. 



