In the course of my examination of the “Challenger” thermometers, I was once surprised by a sudden bell-like vibration of the massive iron cylinder (nine inches thick) in which the apparatus to be tested was enclosed. This was due to the giving way of the protecting-sheath of one of the thermometers, at a pressure of a little over four tons weight on the square inch. When the pressure was let off, the bulb of the thermometer was found to have been crashed to an almost impalpable powder. I then determined to make a set of experiments with the view of finding under what amount of distortion glass gives way. For this purpose I employed the smaller compression apparatus which I have already described to the Society. Its capacity is about ten cubic inches only, so that a couple of strokes of the pump produce a pressure of three to four tons weight.