174 MR. J. H. GRAY ON A METHOD OF DETERMINING 
The wire is now completely enclosed in this tube, and loosely surrounded by cotton 
wool, 
The thermometer is placed in the hole in the ball, and a little water put in to fill 
up the hole. The water-jacket may then be raised so as to surround the ball, and 
the top of the water-jacket must be covered with two or three sheets of paper, holes 
haviug been cut in these, so as to admit the wire and thermometer. 
It was found best to make these preparations a few hours before the actual test 
was begun, so as to allow the system to take up a permanent temperature. When it 
has been ascertained that this temperature has been reached, a reading is taken from 
the thermometer in the ball. The thermometer used was most carefully made and 
calibrated for the tests by Mr. Orro Mtttisr, of Glasgow. The whole length of the 
stem is 15 centims., and it is marked off to read twentieths of a degree from 9° to — 
20° C. Each division is a half millimetre, so it is perfectly easy to read to one- 
fortieth of one degree. 
Before beginning the test, the water-jacket is lowered, and a vessel containing ice 
and water raised so as to cover a part of the ball. By this means the temperature of 
the ball is lowered by 6° or 7° below that of the air. While this is being done, the 
boiling water is poured into the box, so as to nearly fill it, and the Bunsen lamp lit. 
The water soon begins to boil rapidly, and the thermometer (which is 12 centims. 
long and reads from 95° to 105° C.) indicates a constant temperature, usually 97° or 
98° C. When the temperature of the ball has been lowered 6° or 7°, the ice and 
water are taken away, and the ball is carefully dried with a soft cloth. The water- 
jacket is again placed so as to surround the ball and the cover put on as before, the 
circulation of water in the jacket having been started. 
It is now only necessary to take readings of both thermometers every half minute. 
The temperature of the hot water will be practically constant, but it is advisable 
to take the readings in case of alteration. The calorimeter thermometer may be read 
till it reaches 20° C. 
It will be convenient to explain here the reason for cooling the ball 6° or 7° before 
starting. In the preceding remarks no notice was taken of the fact that there will 
be radiation to or from the surface of the ball unless the latter is at the same tempe- 
ture as its surroundings. It would be impossible to allow for this by calculation, as 
the surface is altered before every test by being heated while the wire is being 
soldered into the ball. For the purpose of getting rid of the necessity of allowing 
for this radiation, the ball is cooled down. 
Let Q, =the quantity of heat which flows along the wire in unit time when the 
_ hot end is at T° and the cool end at above the temperature t of the air and water- 
jacket, and let Q, = the quantity when the cool end is & below that of the air. 
Then, assuming that the conductivity does not change very much through 26°, and 
that «, the loss by radiation in unit time when the ball is 6° above the air tempera- 
