In the first engines which I built in Newark the governor had a more or less uncomfortable action. This annoyed me exceedingly. It did not sensibly affect the running of the engine, but was a drawback to the appearance of the engine in motion. I was utterly at a loss how to account for it, so I finally determined I would solve the problem by a comparison of two engines of the same size. One of these was the smaller engine for the rolling mill at Troy, where the action of the governor was quite satisfactory; the other was an engine I had made for the Newark Lime and Cement Company, in which the action of the governor was very unsatisfactory. After some weeks of comparison I gave the problem up: I could get no light on the subject. Soon after I had occasion to go to Troy and found my smaller engine running at double its former speed or at 224 revolutions per minute. Mr. Robert W. Hunt, the general superintendent, informed me that they planned to employ this speed when rolling steel to finish at very small sizes, which they were then doing for the first time. The action of the governor which had before been so perfect was now most abominable; the counterpoise flying up and down furiously between the extreme points of its action. I told Mr. Hunt that something was hindering the action of the governor, and asked him if he would have an examination made and let me know what he found. A few days after I received a letter from him saying he had found nothing at all, but he added that that order had been completed and the engine was running at its old speed, and the governor was working as well as ever. In an instant the truth flashed upon me; it was the inertia of those polished cast-iron disks on the rocker-shaft which I had thought so much of that caused all the trouble. This inertia, increasing as the square of the speed, had offered four times the resistance to the reversing of their motion when the speed of the engine was doubled, and the pressure of the link which was necessary to overcome this resistance held the block fast. The governor could not move it until it had accumulated sufficient force by change of its speed; then it moved it too far, and so it was kept in constant violent motion from one end to the other of its range of action. I was thoroughly ashamed of myself that when I had made the subject of inertia a study for years this action should have been going on so long, the most prominent thing before my eyes, and I never saw it. I had use enough at once for my new insight as will appear.