Fem] 
XXII. 
USEFUL METHODS IN TEACHING ELEMENTARY PHYSICS. 
By J. JOLY, Sc.D., F.B.8. 
[Read Decemzzr 20; Received for Publication DecemBEr 22, 1893; Published 
Frpruary 12, 1894. ] 
l.—The Barometer—Boyle’s Law—Air Thermometer— Cooling of a 
gas upon expansion—Flotation of Bodies. 
Tue subject of elementary scientific education is of so much 
importance that I do not think it out of place to bring before 
the Society methods I have found of use in teaching elementary 
experimental physics. 
The Barometer.—It is a good plan to ask a student who seems 
in a difficulty, what it is that he does no¢ understand, and keep a 
careful note of such answers, for it is beyond the power of most 
teachers to recall their own difficulties when they were beginning. 
Beginners commonly find difficulty in understanding how the pres- 
sure of the atmosphere upon the surface of the mercury in the 
bath supports the column of mercury in the barometer. It isa very 
justifiable difficulty, in a sense, and it is best to put it that the 
bath is not essential at all, save to keep air from going up the tube 
and to supply mercury ; and to show the student what happens 
when the tube is slowly lifted till it leaves the bath. The next step 
is to convince him that the atmospheric pressure had really been 
supporting the mercury all along. In this it is only requisite to 
take a barometer tube of sufficiently fine bore (about 2mm.) and 
pour in some 72 cm. of mercury. Invert it, and the mercury 
hangs up in the tube. Why? ‘Then add a little more mercury, 
and when the then prevailing barometric length is reached, the 
column begins to sink down in the tube. Just when it is going 
to fall out, dip it into the bath. Now the column is secure, and 
it is plain to the pupil that dipping it in the bath cannot alter 
the atmospheric pressure which supports it, but the pressure must 
now be transmitted through the mercury in the bath. 
This experiment may be shown to a whole class by the follow- 
ing arrangement :—A tube of thin glass about 1 cm. bore and 
