THE MEASUREMENT OF STRESS BY THERMAL METHODS. 233 
motor L, and provided with an extension barrel M, heated by a gas flame N. The 
rotation of the pump caused water to be drawn in at the eye of the pump from the 
pipe I’, and to be discharged through the perforated partition O into the encircling 
heater M. From thence it flowed through the rising pipe J and the specimen, as 
_ indicated by arrows. A very vigorous circulation was maintained, and the pipes were 
thickly lagged, so that there was practically no difference of temperature between the 
end points of the tube under measurement. The readings of the thermometers were 
found to be practically coincident at all temperatures, and therefore in the tabulated 
results one set of temperature readings is omitted. The alteration in the length of 
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the tube with increase of temperature was determined by aid of a Hwing extensometer 
of the original pattern. The instrument (fig. 2) consists of a pair of clips A B, secured 
to the tube by set screws C; the upper clip carries a frame D, provided with a 
calibrating screw P and a reading microscope F' focussed on the edge of a thick wire 
W carried by the lower clip. Any alteration in the length of the tube causes a 
movement of the wire relative to the microscope, and by aid of a glass scale inside 
the eye-piece the alteration in leneth can be determined. The dimensions of the 
instrument were such that a movement of 0°00002 inch corresponded to one unit 
of the scale, and the micrometer screw enabled a calibration of the scale to be effected 
while the instrument was attached to the specimen —a great advantage with an 
extensometer. The construction of the apparatus permitted a thick layer of lagging K 
to be applied. It was proposed at first to surround the specimen by a water-jacket, 
fed from the circulating system, to ensure the temperature of the tube being absolutely 
uniform throughout, but this was not done, as the lagging was found to be very 
efficient, and the extra complication did not seem warranted, particularly as only 
