THE INCORPORATORS 163 
to the study of astronomy, and had charge of the Friends’ Observ- 
atory on Cherry Street, Philadelphia, until 1856. He entered 
the University of Pennsylvania, and was graduated from the 
medical department. Afterwards he removed to Sharon Hill, 
Pennsylvania, and engaged in the practice of medicine. He 
still devoted much of his time to astronomy, and wrote many 
valuable papers relating to that branch of science. These were 
published in the T'ransactions of the American Philosophical 
Society, of which Dr. Longstreth had been a member since 1848. 
He was of a retiring disposition and declined public office. For 
forty years, however, he served on private and public educational 
boards. 
DENNIS HART MAHAN 
Born, April 2, 1802; died, September 16, 1871 
Dennis Hart Mahan was born in the city of New York, but 
his parents soon moved to Norfolk, Virginia, where his boyhood 
was spent. He was brought up with the idea that he would be a 
physician, but having a talent for drawing, and learning that 
this was taught at West Point, he sought and, through the good 
offices of a friend of the family, obtained admission into the 
Military Academy. From this institution he was graduated in 
1824, at the head of his class, which numbered thirty-one students. 
In his third year at the Academy he was appointed Acting Assist- 
ant Professor of Mathematics. Following graduation he became 
a lieutenant in the Corps of Engineers, and after holding the posi- 
tion of instructor for two years in the Academy was sent to Europe 
to study engineering works and military institutions. In France, 
by special permission of the Government, he studied for more 
than a year in the military school at Metz, and became associated 
with many prominent French military engineers and artillerists, 
and was often the guest of the family of Lafayette. 
He returned to America in 1830 and was detailed as acting 
professor at West Point. Two years later he vacated his com- 
mission in the Engineer Corps, and became Professor of Civil and 
Military Engineering. In this important position he remained 
