275 



which parallax most affects the angle of position is when the great 

 circle passing through both stars, passes also through the pole of 

 the ecliptic. But to ascertain how far any particular star is or is 

 not favourable to the application of the method, and the times of the 

 year when it ought to be observed, it is necessary to make a calcu- 

 lation, the mathematical principles of which the author explains ; 

 and includes, in proper formulae, which he then applies to the for- 

 mation of a list of about 70 stars, as a specimen. 



He concludes by observing, that by a certain mode of using the 

 double image-position micrometer, first explained to him by Captain 

 Kater, the application of this method ceases to be limited to close 

 stars ; and that stars of any moderate distance, otherwise favourably 

 situated, may be equally well subjected to examination with nearer 

 ones ; and that thus the range of objects placed within our power 

 becomes unlimited. 



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. Communicated February '2, 

 1826. Read April 19. 1826. [Phil. Trans. 1826, p. 281.] 



The author first observes that an opinion is generally prevalent, of 

 a decided increase in the average duration of human life in many 

 parts of Europe ; but he yet regards it as probable that this improve- 

 ment has been much exaggerated, partly on account of the limited 

 number of persons on whom the observations have been made, and 

 partly from erroneous views respecting the profits of assurance com- 

 panies. 



He then examines the evidence on which this opinion rests, and 

 gives a comparative statement of the annual average of mortality, 

 the mean term of full life, and the mean age of mankind, according 

 to a great number of different authorities ; and considers that a pro- 

 longation of life to the extent of one year in eight, is a much fairer 

 estimate than one in three, which some have maintained, even on 

 the limited grounds of the experience on which they have reasoned. 



Another mode, he observes, of easily appreciating the regularity 

 and analogies of tables is the construction of a curve in which, the 

 abscissa representing the age, the ordinate shall represent the corre- 

 sponding decrements of life. This he accordingly does, and its in- 

 spection he observes is sufficient to render us suspicious of the ac- 

 curacy of the Carlisle tables ; while he considers a combination of 

 these and the Northampton tables, and the London parish registers, 

 as likely to give the fairest estimate. 



After commenting on the various documents before him, the author 

 next proposes the formula mentioned in the title, which consists of 

 terms having respectively a preponderating influence in infancy, in 

 youth, in middle age, and in old age. 



This is followed by a series of numerical documents, and the 

 ralues of the formula calculated for each year ; and the curve corre-' 



T 2 



