MESSRS. EDITORS :In reading the discussion on hot air engines, published on page 310 of the present volume of the SCIENTIFIC AMERICAN, I noticed the question was asked, "Why its use was discontinued in the Dundee Foundry ?" I was working there for the greater part of the time that the second hot air engine was running. A common beam engine had been tans-form ed into the hot air engine by the addition of the heating air pump a nd other requisites. The great trouble with the air engine was the cracking of the' ' heaters and their expense. They weighed about four tuns each. They had to be molded in loam, the same as the cylinder of an engine faced up in the lathc, and everything done that was required for fitting a cylmder head. Some would last for two months, others a year ; and the time required for taking one out and making another was from a week to ten days. Stirling left the Dundee Foundry at the time it was discontinued, and it passed into the hands of Gourlay, Mudie & Co., who were not interes ted in it. The great expense entailed for new heaters was the cause of' its discontinuance in the Dundee Foundry. The same cause led to itsS discontinuance in the factory at Dun dee, viz., the expense of replacing the heaters and the stoppage of the factory so often for repairs. The hot air engine in the foundry had a very unsteady motion; so much so, that it was necessary to put new couplings on all the shafting when they put in the steam engine. JAMES GUTHRIE. Boston, Mass., Nov. 30, 1860. [The unsteady motion of the air engine in the Dundee Foundry referred to by our correspondent has also been experienced with the air engines on this side of the Atlantic. We have never heard of a large hot air engine which did not experience great difficulty and expense fi'om the cracking of the large heaters necessary to warm the air ; but on a small scale, such as a two or five-horse power engine, this di fficulty may be overcome to a certain ex ten t.