SECTIONAL TRANSACTIONS.— A. 331 



described. The results are given of a series of experiments made to de- 

 termine the yield of liquid helium obtained from different initial conditions 

 of pressure and temperature ; in addition some measurements on the 

 equation of state and the specific heat of gaseous helium in so far as they 

 affect the expansion method are reported. 



General Discussion on Low temperature physics. 



Dr. J. Hartmann. — The Acoustic Jet Generator (12.15). 



The acoustic air-jet generator, introduced to the members of the British 

 Association at the Oxford Meeting, 1926, has since then been made the sub- 

 ject of investigations as to its modus operandi, its performance, its techniques 

 of measurement, etc. It now represents, undoubtedly, the most effective 

 means available for the production of large power high frequency waves 

 in air, and would seem to open up a wide field of scientific and technical 

 applications. The communication deals mainly with the explanation of 

 the peculiar phenomenon underlying the working manner of the generator. 

 This phenomenon and the air vibrations produced in the apparatus are 

 illustrated through a number of photographs taken by means of the method 

 of striae and by a special method developed jointly with the generator 

 (the method of the Riemann mirror). 



Demonstration of the generator proper together with some of the effects 

 of the waves. Display of a collection of photographs from the researches 

 on the generator. 



Dr. H. Jeffreys, F.R.S. — Temperature conditions within the earth's 

 crust (12.30). 



Reports of Committees (12.40). 



Afternoon. 



Mr. H. L. P. Jolly. — Terrestrial magnetic bearings and their practical 

 uses {with special reference to a new Magneto-Theodolite) (2.15). 



The usefulness of the magnetic bearing of an object depends upon the 

 precision with which the magnetic declination at the time and place is known 

 and upon the precision of the instrument with which the observation of the 

 bearing is made. A new Magneto-Theodolite, incorporating the Smith 

 fluid immersion (inverted pivot) mounting and a quick change-over from 

 theodolite operation to magnetic reading and vice versa by means of penta- 

 prism interposed in the telescope, is described. Tests have shown that the 

 pivot is sufficiently free from friction to allow the instrument to follow 

 changes of declination within fifteen seconds of angle or less. When 

 working in sunlight or other conditions of rapid temperature change, con- 

 vection may cause irregular movements of about a minute of angle. 



The instruments work in pairs by simultaneous observation. Relative 

 declination at a series of points may thus be quickly determined. If the 

 distribution of declination in the area is already known, one instrument 

 can be used to give true bearings from the magnetic bearing whilst the other, 

 situated at a base station not very distant, makes simultaneous observations 

 to control the time changes. 



