a new addition to the Slide- Rule. 53 



For the sake of brevity let us name the scales in a rule of 

 the ordinary " Gravet " type thus 



D 

 becoming when the slide is inverted, 



A 



D 



Then with the slide in the inverted position the corre- 

 sponding pressures and volumes of an isothermal can be read 

 off directly on the scales A and $j. 



In the case of an adiabatic curve, however, we have an 

 expression of the form 



pv?= const., 



where 7 has a value 1*4 more or less according to the physical 

 properties of the gas or mixture of gases we are dealing with. 

 In order to deal with an expression of this form, it is neces- 

 sary to provide some ready means of dividing the scales A 

 and q proportionately to the value of 7 which corresponds to 

 the division and multiplication of the respective logarithms 

 of the quantities dealt with, in the proportion of the indices 

 of p and v, that is 1 : 7. 



This proportionate division of the scales is effected in the 

 new Cursor by a radial index-arm, indicated in fig. 1 by 

 lines / and g : this is arranged to swing about a stud which 

 is itself carried on a sliding bar running in guides at right 

 angles to the rule. All readings are taken at the points of 

 intersection of the line with the edges of the slide. In the 

 actual instrument the radial arm is made of a transparent 

 slip of celluloid on which is scratched a straight line true to 

 the centre of the stud-hole. The back view shows (fig. 2) 

 the transverse bar graduated to give the indices to which 

 it is desired to work. In fig. 2 this transverse bar is shown 

 set to 1*408, the 7 of a perfect gas. 



A few words may be added with regard to the method of 



