Prof. Ch. V. Zenger on a New Steam-gauge. 345 



and unalterable in its working than the spring gauges now com- 

 monly used for steam-boilers. 



In the first place, it is a great defect in the common air-mano- 

 meter that the divisions on the manometric tube diminish rapidly 

 at high pressures, and consequently the reading becomes less 

 and less accurate the higher the pressure. The new steam-gauge, 

 on the contrary, possesses the same degree of accuracy at all 

 pressures, and even enables us to make the accuracy of reading 

 greater at high pressures. 



Another serious defect of the air-manometer is the liability to 

 rupture of the narrow column of mercury, consequent upon the 

 sudden shutting off or turning on of the steam. This is entirely 

 avoided in the present instrument by the use of two closed ves- 

 sels communicating with each other only by very narrow capil- 

 lary tubes. Finally, the small column of mercury enclosed in 

 the glass tube of common air-manometers is subject to capillary 

 depression, and to the disturbing effects of heat upon the air- 

 bulb and upon the mercury. In the instrument now to be de- 

 scribed, it is sought to avoid these defects by not using capil- 

 lary tubes for the manometer, and by disposing the air and mer- 

 cury in such a way as to make the effect of heat sensible. 



The construction of the apparatus will be plainly seen from 

 the annexed diagrams*, representing the apparatus in three dif- 

 ferent sections. The air- tube of the manometer consists of a 

 series of tubes of equal lengths but different diameters, melted 

 together before the lamp, and ending at the top in a glass bulb. 

 The lower end is connected by an air-tight screw-joint with the 

 first of two iron vessels containing mercury or some other liquid, 

 and communicating with each other only by a very narrow capil- 

 lary channel. The manometric tube is sealed at the bottom ; 

 but there are two fine capillary openings through the side at 

 points below the surface of the mercury or other liquid contained in 

 the two iron vessels. Hence the communication of pressure from 

 the steam or other compressed gas whose pressure is to be mea- 

 sured, and which presses directly upon the surface of the liquid 

 in the second iron vessel, can only take place through a system 

 of two successive capillary channels ; and the resistance which 

 these channels oppose to the motion of the mercury by which 

 they are filled makes it impossible for sudden changes to occur 

 in the height of the manometric column, and thus entirely pre- 

 vents the division of the column or the entry of steam or gas 

 into the manometer. 



The capacities of the globe and of the several tubes which com- 

 pose the manometer are so adjusted that they decrease in the 



* [For the loan of these diagrams we are indebted to the kindness of the 

 Proprietors of * The Engineer.' — Eds. Phil. Mag.~) 



