SAMUEL PIERPONT LANGLEY. 
15 
from Thomas G. Hodgkins, came during the administration 
of Mr. Langley. 
Among his many notable addresses was that delivered in 
1888, as the retiring President of the American Association 
for the Advancement of Science, under the title of “A His- 
tory of a Doctrine,” this doctrine being the views concern- 
ing radiant energy. The address is a charming one in 
every respect. — as an historical investigation, as a summing 
up of results obtained, as a literary document, and as a 
prophecy. Some of the phrases are worthy of a great phil- 
osophic mind. “We have perhaps seen,” he declared, “that 
the history of the progress of this department of science is 
little else than a chapter in that larger history of human 
error which is still to be written.” And yet there is no 
pessimistic note in it, for he asks the question, “Shall we say 
that the knowledge of truth is not advancing?” and he re- 
plies to this query, “It is advancing, and never so fast as 
today; but the steps of its advance are set on past errors, 
and the new truths become such stepping-stones in turn.” 
To this same time belong other papers of great general 
interest, notably that on “The Observation of Sudden Phe- 
nomena,” which will have a certain value even for the 
physiological psychologist, although designed for the as- 
tronomer primarily and containing descriptions of the per- 
sonal-error machine invented by Mr. Langley ; and also 
another paper on “The Cheapest Form of Light,” this study 
being based upon an examination of the radiation of the fire- 
fly, iand showing that it is possible to produce light without 
heat other than the light itself, and that this is actually 
effected now by nature’s processes. 
I am brought, however, to another field of scientific work 
in which Mr. Langley engaged, and with which his name 
has been identified during the past fifteen years, the subject 
popularly known as flying-machines, and which he de- 
nominated aerodynamics. Mr. Langley came before the 
scientific world and the public generally on this subject first 
in a very brief communication to the Academy of Sciences 
of the Institute of France, in July, 1890; second by the pub- 
