is working.  To this arrangement is connected a powerful brake which stops the machine in a few revolutions.  It will be seen that the brake is applied as the belt is slackened for stopping the machine.  For planishing pipes or tubes a long wrought iron mandrel is provided mounted on two cast iron carriages, each having four flanged wheels for running on rails.  The hammer is arranged so that tubes 4 feet in diameter can be worked for planishing plates.  A pallet is fastened on the top of one of the mandrel carriages, Figs. 5 to 8 showing the details of the carriages.  The general dimensions are:  Distance between pillars, 6 feet; height under girder, 5 feet; height from ground to top of mandrel, 4 feet 13/4 in.; and length of stroke, 5 in.  This machine is capable of delivering 500 blows per minute.  The constructors are Messrs. Thwaites Brothers, Limited, Bradford, Yorkshire.

[Illustration:  Fig. 1.]

[Illustration:  Suspension pneumatic power hammer.]

* * * * *

SIBLEY COLLEGE LECTURES.—­1890-91.

By the Cornell University non-resident lecturers in mechanical engineering.

Compressed air production.

By Wm. L. Saunders, C.E., of New York.

I cannot but realize as I stand before you that I would be very much more at home were I in your midst.  I feel but little older and so very much less wise than when I sat in the class room an undergraduate of the University of Pennsylvania, that I trust I may expect you to give me this afternoon, not only your attention, but your sympathy.

The present situation is not without suggestions of my own experience.  I recall a lecture in the ordinary course, given by our professor of mining, whose struggles with the English language were quite as conspicuous as were our efforts to tell what he was driving at.  He was describing an ordinary windlass hoist used at the shaft of a mine.  He said “There is a windlass at de top of de shaft around which is coiled a rope, on de two ends of which is fastened two er—­er—­pans, one of which is a bucket and de oder a platform.”  I mention this because I shall ask you to attribute my shortcomings in this lecture, not so much to my lack of familiarity with my native tongue, as to—­well, because I was not educated at Cornell University.

We all know what free air is.  You who are privileged to live upon these beautiful hills overlooking Ithaca and the lake, doubtless know more about free air than we do who are choked in the dusty confines of New York City.  Compressed air is simply air under pressure.  That pressure may be an active one, as in the case of the piston of an air compressor; or passive, as with the walls of a receiver or transmission pipe.  It is usual to define compressed air as air increased in density by pressure, but we know that we may produce compressed air by heat alone.  A simple illustration of this is the pressure which will blow a cork from an empty bottle when that bottle has been placed near the fire.  Here we have pressure, or compressed air, in the bottle produced by heat alone.