Capacities of Blowpipes.
—It has been said, a blowpipe with a 1⁄8-in. air jet, if worked with an air pressure of 10 oz. per square inch—that is, 15 in. of water—will braze up to about 1⁄2 lb. total weight; or in other words, will securely unite two pieces of brass each weighing 1⁄4 lb. With the same pressure a 1⁄4-in. bore air-jet will braze a total weight of about 2 lb., and so on in proportion. It will be understood that the air jet is measured at the point at which the blast leaves the air tube, whilst the area of the gas supply is that of the annular space between the two tubes. When the air tube is thus carried inside the gas tube (see [Fig. 69]), the tool appears to be much larger than it really is, and this accounts for the fact that a 1⁄2-in. size blowpipe with the air tube fixed outside the gas supply is just as effective as one of the 3⁄4-in. size which carry the air tube inside the stem. All indiarubber tubing must be perfectly smooth inside, for if it is wired or in any way rough, the resultant friction will cause a loss of pressure. It should also be of as large a bore as is convenient.