On measuring the relative Tension of Electric Currents. 339 



single life is worth cf) (/) or <^ {hk)^ the value of an annuity 

 on the joint lives of B and C. 



The various causes of error in the preceding, arising from 

 the change in the values of the constants, will readily pre- 

 sent themselves to those who have studied the subject. 



It is not, of course, necessary that the progression of powers 

 should be precisely that of Mr. Gompertz, and the following 

 theorem may easily be demonstrated. If a„ be the chance of 

 a life living n years, and if 



«„ = {a^)Pn, 



where p^ is not a function of a, then the annuity on any 

 number of lives is not altered in value if, instead of any part 

 of those lives, a single life of equivalent value be substituted. 



B 



LI. On the Use of a Secondary Wire as a Measure of the 

 Relative Tension of Electric Currents. By John W. Draper, 

 M.D.^ Professor of Chemistry in the University of New York ; 

 late Prof, of Physical Science in Hampden Sydney College^ 

 Virginia. 



[Continued from p. 279, and concluded.] 



^EFORE however we can go further in the study of these 

 conditions of tension, or attempt to show that the arrange- 

 ment of Volta, and a single pair under the influence of a long 

 or thin wire, are, in point of fact, alike in principle, it is ne- 

 cessary that we should understand the nature of the different 

 disturbing actions that may arise in the generating cells of 

 the electromotor. 



I took a zinc plate 7 inches long and 3 wide, and a corre- 

 sponding copper : the surface of the former was amalgamated 

 and the latter brightened. The plates were fixed at an im- 

 movable distance from each other, and immersed in a jar 

 containing 34- ounces of water. Sulphuric acid was then 

 added by half drachms successively. 



Table E. 



