TRANSACTIONS OF THE SECTIONS. Ill 



The sills against which the gates are made to fit, may be regulated so as to dis- 

 charge the largest flood of a river without allowing the gates to open further than 

 necessary, or in rivers where it is expedient to remove every obstruction to the cur- 

 rent, to prevent inundation, or to pass very large bodies of water, the sills may be 

 so arranged that the gates should assume a nearly horizontal position, opposing 

 little more resistance to the water than the thickness of the material of which the 

 gates may be composed. 



On a New Steam-engine teorked with three kinds of Pressure, viz. Action of 

 high-pressure Steam, the Expansion of Steam, and the Atmospheric Press- 

 ure caused by its Condensation. By Mr. Shaw. 



The description was illustrated by models and drawings. The lower part of the 

 piston fits the cylinder and is steam-tight ; the upper part or plunger, in the form of 

 a hollow cylinder, is longer than the cylinder, and passes through a stuffing-box in 

 its cover. The piston rod rises through the middle of the plunger, and is connected 

 with the parallel motion ; the valve is a modification of the single slide, the upper 

 passage leading to the upper part of the cylinder, the middle to the lower part of the 

 cylinder, and the lowest to the condenser. When the piston is descending, the valve 

 admits steam to the annular space between the plunger and cylinder, when, the 

 middle and lower passages being open, the direct pressure of the atmosphere upon 

 the plunger, the high-pressure steam upon the annulus, and an additional pressure 

 of one atmosphere upon the annulus from the vacuum beneath, concur to produce 

 the down stroke ; during this the steam is cut off from the cylinder, and the com- 

 munication between the cylinder and condenser is shut ; the steam, which before 

 occupied the annulus, now acts against the plunger for the ascending stroke with 

 the whole force of expansion, the annular surface of the piston being then passive. 



A model and drawings of Mr. Shaw's hydraulic engine were also exhibited. In 

 this engine, the want of elasticity in water, which formed a great objection to the 

 application of that fluid to a piston, is supplied by the elasticity of air in a chamber 

 communicating with the cylinder ; it is well fitted for situations where the fall is 

 great, but the supply too small or the space too limited for the use of a water-wheel, 

 as in mines. 



On a dry Gas-Meter. By Mr. Clegg. 



It acted on the principle of the differential thermometer, in which a difference of 

 temperature between two bulbs partly filled with alcohol, was shown by the rising 

 of the spirit in one and depression in the other. He had taken advantage of this 

 principle by suspending two little glass vessels, partly filled with alcohol and con- 

 nected by a tube, and by passing the gas over heaters it warmed one of these bulbs, 

 and the spirit was driven into the other, which, becoming the heaviest, swung to the 

 bottom of the arc in which the vessels vibrate ; here it in its turn becoming warm, 

 was emptied of its spirit, and thus becoming lightest, was in turn displaced ; the 

 continuance of these oscillations marked the flow of gas, and being registered by the 

 usual train of wheel-work, the number of vibrations had been proved, by many 

 careful experiments at all seasons of the year, to afford an accurate measure of the 

 quantity of gas. 



On the Tliames Tunnel in its completed Condition. By Sir M. I. Brunel. 



A representation of the machinery, designated the shield, by the agency of which 

 the excavation had been effected and the structure simultaneously made up, was 

 exhibited. 



The shield, standing as it does between the ground and the constructed portion, 

 may be considered as the chief agent, on the functions of which everything essen- 

 tially depends. 



The Thames Tunnel, he remarked, including the shafts at each end, is a quarter 

 of a mile in length, its breadth is 37 feet 6 inches to 38 feet, and the height of the 

 excavation 22 feet 6 inches ; presenting therefore a sectional area of 850 feet. 



The thickness of the ground over the structure, or over the shield, is 14 feet at the 



