332 Mr. J. Prescott on the 



with no difference of potential between them (" "). The 

 next position ("1") puts in the first cell, and thereafter 

 the potential difference between the terminals rises by 

 approximately equal steps of about 1*4 volts as the handle 

 is rotated, until all the cells are included in the circuit. 



The outside dimensions of the box shown in fig. 2 are 

 22 cm. square by 9'5 cm. high. Cells of about twice the 

 capacity of those mentioned above can also be obtained. 

 These larger cells are 5*5 cm. high and 1*9 cm. in diameter, 

 and twenty of these can conveniently be arranged in a box 

 25 cm. square by 10*5 cm. high. The larger cells are 

 recommended as having a longer life than the small ones, 

 but the arrangement is not quite so compact. From the 

 diagram (fig. 3) it will be seen that any cell can readily 

 be slipped out of the springs which hold it. It is thus easily 

 replaced by a new one — there is no soldering to be done or 

 screw connexion to make. 



The Physical Laboratory, 

 Royal Holloway College, 

 Englefield Green. 



XXXIV. On the Motion of a Spinning Projectile. By J. 

 Prescott, M.A., Lecturer in Mathematics at the Manchester 

 School of Technology*. 



On the Motion of a Spinning Projectile. 

 1. TN order to make a beginning of the problem of the 



_i_ motion of a projectile, we need a formula giving the 

 resistance of the air to a body moving through it. Bash- 

 forth found it convenient to assume that the resistance 

 varies as the cube of the velocity, while he pointed out that 

 the main reason for his choice of this law was that it was 

 the easiest to work with mathematically. Actually the 

 cubic law is very far from the true law except over two very 

 small ranges of velocity. Bashf orth's method was to express 

 resistance in the form K V V 3 , V being the velocity of the 

 projectile, and K v a variable which is treated as a constant 

 in his mathematical theory, the errors introduced in conse- 

 quence of this incorrect assumption being kept small by his 

 dividing the path into small portions and using a different 

 K in each portion. 



2. From Bashforth's own results, however, which are 

 based on observations, it is clear that the resistance of the 

 air is much more nearly proportional to the square of the 



* Communicated by the Author. 



