

CHIMNEY 773 



length, increased by four times its diameter. Thus, for pottery, its expression is 

 2-06 . ; D being the diameter, and t the length of the chimney. 



L + 4D 



A pottery chimney, 33 feet high, and 7 inches in diameter, when the excess of its mean 

 temperature above that of the atmosphere was 205 Fahr., had a pressure of hot air equal 

 to 11*7 feet, and a velocity of 7'2 feet per second. By calculating from the last formula, 

 the same number very nearly is obtained. In none of the experiments did the velocity 

 exceed 12 feet per second, when the difference of temperature was more than 410 Fahr. 

 Every different form of chimney would require a special set of experiments to bo 

 made for determining the proper factor to be used. 



This troublesome operation may be saved by the judicious application of a delicate 

 differential barometer, such as that invented by Dr. Wollaston ; though this instrument 

 docs not seem to have been applied by its very ingenious author in measuring the 

 draughts or ventilating powers of furnaces. 



If into one leg of this differential siphon, water be put, and fine spermaceti oil into 

 the other, we shall have two liquids, which are to each other in density as the numbers 

 8 and 7. If proof spirit be employed instead of water, wo shall then have the relation 

 of very nearly 20 to 19. In experiments made on furnace draughts with the instru- 

 ment in each of these states, it is found that the water and oil siphon is sufficiently 

 sensible : for the weaker draughts of common fire-places the spirits and oil will be 

 preferable barometric fluids. 



To the lateral projecting tube of the instrument, as described by Dr. "Wollaston, it 

 was found necessary to attach a stop-cock, in order to cut off the action of the chimney, 

 while placing the siphon, to allow of its being fixed in a proper state of adjustment, 

 v,-ith its junction-line of the oil and water at the zero of the scale. Since a slight de- 

 viation of the legs of the siphon from the perpendicular changes very considerably the 

 line of the level, this adjustment should be made secure by fixing the horizontal pipe 

 tightly into a round hole, bored into the chimney stalk, or drilled through the furnace 

 door. On gently turning the stop-cock, the difference of atmospheric pressure cor- 

 responding to the chimney draught will be immediately indicated by the ascent of the 

 junction-line of the liquids in the siphon. This modification of apparatus permits the 

 experiment to be readily rectified by again shutting off the draught, when the air will 

 slowly re-enter the siphon ; because the projecting tube of the barometer is thrust into 

 the stop-cock, but not hermetically joined ; whereby its junction-line is allowed to 

 return to the zero of the scale in the course of a few seconds. 



Out of many experiments made with this instrument, it will be sufficient to describe 

 a few, very carefully performed at the breweries of Messrs. Trueman, Hanbury, and 

 Buxton, and of Sir H. Meux, Bart., and at the machine factory of Messrs. Braithwaito 

 by Dr. Ure ; in the latter of which he was assisted by Captain Ericssen. In the first 

 trials at the breweries, the end of the stop-cock attached to the differential barometer 

 was lapped round with hemp, and made fast into the circular peep-hole of the furnace 

 door of a wort-copper, communicating with two upright parallel chimneys, each 18 

 inches square and 50 feet high. The fire was burning with fully its average intensity 

 at the time. The adjustment of the level being perfect, the stop-cock orifice was opened, 

 and the junction level of the oil and water rose steadily, and stood at li inch corre- 

 sponding to ^f = 0'156 of 1 inch of water, or a column of air 10'7 feet high. This dif- 

 ference of pressure indicates a velocity of 26 feet per second. In a second set of ex- 

 periments, the extremity of the stop-cock, was inserted into a hole bored through the 

 chimney stalk of the boiler of a Boulton and Watt steam-engine of twenty-horse 

 power. The area of this chimney was exactly 18 inches square at the level of the 

 bored hole, and its summit rose 50 feet above it. The fire-grate was about 10 feet 

 below that level. On opening the stop-cock the junction-line rose 2 inches. This 

 experiment was verified by repetition upon different days, with fires burning at their 

 average intensity, and consuming fully 12 Ibs. of the best coals hourly for each horse's 

 power, or nearly one ton and a third in twelve hours. If we divide the number 2^- 

 by 8 the quotient 0-28 will represent the fractional part of 1 inch of water supported 

 in the siphon by the unbalanced pressure of the atmosphere in the said chimney ; 

 which corresponds to 19^ feet of air, and indicates a velocity in the chimney current 

 of 35 feet per second. The consumption of fuel was much more considerable in tho 

 immense grate under the wort-copper than it was under the steam-engine boiler. 



In the experiments at Messrs. Braithwaite's factory, the maximum displacement of 

 the junction-line was 1 inch, when the differential oil and water barometer was placed 

 in direct communication with a chimney 15 inches square, belonging to a steam boiler, 

 and when tho fire was made to burn so fiercely, that, on opening the safety-valve of 

 tho boiler, the excess of steam beyond tho consumption of tho engine rushed out with 

 such violence as to fill the whole premises. The pressure of one-eighth of an inch of 

 water denotes a velocity of draught of 23'4 feet per second. 



