'96 Mr. Patten'' s Air Fump, Gazometer, ^c, 



derE, the water in the cyhnder F is expelled between the 

 two outer cylinders at the aperture L; and its place sup- 

 plied through the valve G, by the gas that was in the cyl- 

 inder E5 now if the cylinder is again filled with gas through 

 D, both it and the cylinder F are full and may be used 

 at the stop cock at H ; that in the cylinder E by pressing it 

 down, and that in the cylinder F by pouring in water be- 

 tween the two outer cylinders. If it is to be used as a 

 Fepys apparatus, remove the cylinder E and its frame 

 work AB, open the valve G by the thumb screw H, and 

 fill the cylinder with water, shut the valve aud introduce 

 the gas at the aperture C through the tube P which has a 

 valve upon it to prevent the water or air from escaping, 

 (shown in fig. 5th,) as the gas comes through P, the water 

 escapes between the two outer cylinders at the tube L, and 

 as this tube is above the top of the cylinder F there will 

 always be a quantity of water upon it that answers instead 

 of the dish in Fepys ; if a jar is to be filled with gas, let it be 

 first filled with water, and inverted over the valve G, turn 

 the screw H and it is instantly done, because the gas in the 

 cylinder F is pressed by the weight of a column of water 

 of the height Ch ; the whole should be well japanned and 

 the cylinder F should not come close in contact with the 

 bottom of the outer cylinder. 



Another instrument 1 take the liberty of sending you isa bal- 

 ance beam, intended to take specific gravities and to weigh ar- 

 ticles of inconsiderable magnitude with a degree of accuracy 

 not easily attainable by the usual method. One great objec- 

 tion to the usual balance is the difficulty of getting the points 

 of suspension of the scales to be at equal distances from the 

 point of suspension of the beam ; another is the friction — ■ 

 this alone is sufficient to prevent the substance to be weighed 

 from containing an exact quantity of matter with the weight 

 used, for suppose a beam to be so nicely constructed as 

 10 turn with the tenth of a grain — now to weigh a hundred 

 grains, that weight will be put into one dish and the sub- 

 stance into the other, but the tenth of a grain must be ad- 

 ded before the scale has turned perceptibly, it therefore 

 exceeds one hundred grains by the tenth of a grain, and 

 the weights can only be equivalents when the index is at 

 rest, which may be anywhere within the quantity required 

 to turn the beam — sufficiently accurate for all common pur- 



