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XX. A formula for expressing the decrement of human life. 
In a letter addressed to Sir Edward Hyde East, Bart. M. P. 
F. R. S. By Thomas Young, M. D. For. Sec. R. S. Com- 
municated February 2, 1826. 
Read April 19, 1826. 
My Dear Sir, 
The investigation of the laws, by which the general mor- 
tality of the human species appears to be governed, is of 
equal importance to the statesman, the physician, the natural 
philosopher, and the mathematician ; and as you have had 
occasion to pay particular attention to the subject, I trust that 
it will not be disagreeable to you to receive the results of an 
inquiry, into which I have entered, for the purpose of appre- 
ciating, if not of reconciling, the many discordant opinions 
that have been advanced, respecting the comparative mortality 
of mankind, at different times, and under different circum- 
stances. 
Of late years, there is little doubt, that, whether from the 
protective effects of vaccination in infancy, or from the in- 
crease of the comforts of the poorer, and of the temperance of 
the more affluent classes of society, or in some measure also 
from the simplification of the practice of physic and surgery, 
there is a decided increase in the mean duration of life in 
many parts of Europe : but it is also extremely probable 
that this improvement has been greatly exaggerated ; partly 
on account of the limited description of the persons on whom 
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