V. HYDROMETEKS. 173 



gas by means of a pipe adjusted in the setting of the apparatus. The pressure, 

 that is, the difference between the two mercury columns, is read by means of 

 a small cathetometer, of which the inillirnetric graduation is turned towards 

 the tubes and the lamp which lights them both. This graduation is reflected 

 in the telescope by means of a total reflecting prism placed before the objective. 

 Thus the level is at once read, and a micrometrical division placed at the focus 

 of the eye-piece shows the ^ of the millimetre. 



673a. Delicate Pressure Gauge. E. A. Cowper. 



The very light pressure required to produce a current through stoves can 

 be shown by the delicate pressure gauge (enclosed in a mahogany case), 

 which has a thin diaphragm exactly one square foot in area, and has a light 

 lever and weights to cause it to bear the pressure brought on its surface, 

 so that the whole pressure of the air on one square foot is exactly weighed 

 with ease. As small a pressure as one hundredth of an inch of water can be 

 measured. 



673b. Multiplying Manometer, for measuring the force of 

 the draught in chimneys and stoves, as well as for the pressure 

 of gas. Dr. K. List, If a gen, Westphalia. 



This apparatus was first constructed by the exhibitor in 1862 for the 

 purpose of meeting the desire of several blast-furnace proprietors resident 

 at Hagen, who wished to be able to measure more accurately the pressure 

 of the blast in their furnaces than was possible with the ordinary manometer, 

 consisting of a curved glass tube, as is mentioned in the description given in 

 the " Zeitschrift des Vereins deutscher Ingenieure," (Journal of the Society of 

 German Engineers) of 1863, vol. vii, p. 493. It consists essentially of a 

 long narrow horizontal and two wider vertical glass tubes, the latter of which 

 are filled to about half their length, and the first up to a long air-bubble, 

 with coloured petroleum. If a suction is effected on one of the vertical tubes, 

 the air-bubble contained in the horizontal tube must, if the vertical tube has 

 a diameter n times as large as the horizontal tube, traverse a distance in 

 the horizontal tube ri 2 times as large as the liquid rises or falls, as the 

 case may be, in the vertical tube ; the sensibility, therefore, can be increased 



71^ 



at pleasure. The space which the index travels is - times as large as the 



difference in the respective heights of the liquid in the two vertical columns. 



The apparatus has been constructed for some years by the exhibitor 

 himself ; the scale indicating in water-millimeters the difference of pressure 

 ,vas prepared by him in the manner described in the Journal of the Society 

 of German Engineers. The apparatus has answered well everywhere. It 

 has been recommended for measuring the pressure of gas by authorities such 

 as Schiele in Frankfort-on-the-Maine and Schilling at Munich. 



673c. Level Manometer, by Gallaud. M. Breguet, Paris. 



673d. Two-fold Control Manometer. 



C. D. Gabler, Hamburg. 



1. The advantage here consists in two independent manometers, acting 

 under quite similar conditions, being combined in one case, whereby not only 

 a greater guarantee of control is obtained, but by the close proximity of the 

 two scales the simultaneous reading of the divisions is facilitated. The 

 upright position of the tubes prevents the collection of impurities out of the 



