18 REPORT OF THE SECRETARY. 



tTrenty-five years. The i^relimi nary labor was the collection and arrange- 

 ment of a full list of stations at which thunder-storms have been recorded, 

 the number of observations at each place, the time of beginning and 

 ending of each storm, and the whole number of thunder-storms occur- 

 ring in each year, and during the whole time at each place. From these 

 data the relative frequency of thunder-storms in different parts of the 

 country can be determined ; also their relative frequency in different 

 seasons and years, as well as the extent of the area over which they 

 occur on the same day. The attending casualties, collected from all the 

 observations made by several hundred observers during a period of 

 twenty-five years, when brought together and classified, will strikingly 

 illustrate the operations of one of the most energetic agents of nature. 



For the preliminary arrangement of the materials preparatory to 

 scientific deductions from them, the Institution has employed Mr. Georg^e 

 H. Boehmer. 



Another series of reductions relative to the meteorology of North 

 America which will occupy the attention of the Institution is that of 

 the discussion of the observations on the barometer in various parts of 

 the United States. This will be commenced as soon as the other series 

 of investigations are completed. 



Comets. — The first research relative to these I have to mention is that 

 relative to a comet of short period or one that returns after a few years. 

 Of these, there are at present six known, namely, Encke's, which has a 

 period of about 3i years; Winnecke's, of between Sand 6 years; Biela's, 

 of 6^ years; Faye's, of 7^ years; Tuttle's, of IS/^ years; and Halley's, of 

 about 76 years. 



All the other comets which have entered the solar system have never 

 been known to return, their orbits having probably, in many cases, 

 been changed by the perturbp^tions of planets in whose vicinity they 

 may have passed. 



The motion of the periodical comets is an object of great interest to 

 the astronomer, as well as to the general physicist, in its relation to 

 the question of the existence of a retarding medium filling interplan- 

 etary spaces, and therefore it is considered an object of much importance, 

 not only to observe their successive positions at their periodic returns, 

 but also to calculate with great precision their orbits as affected by 

 planetary perturbations. At a meeting of European astronomers in 

 August, 1873, the work of discussing all the observations which have 

 been made on four of these comets was parceled out among the conti- 

 nental astronomers, Halley's comet having previously received great 

 attention from the English and other astronomers ; and the fifth, that 

 of Tuttle, discovered by an American astronomer, was left to be worked 

 out in this country. This task has been undertaken at the expense of 

 the Smithsonian Institution, under the direction of Prof. Ormond Stone, 

 late of the National Observatory, and now professor of astronomy in the 



