RECENT ORIGINAL WORK AT HARVARD. 489 



head of original investigation, the observations constantly taken in 

 connection with the Observatory Time Service resolve themselves into 

 mere routine work. An immediate and practical benefit is conferred 

 by this Time Service, the signals of which reach Bangor, Lcnnoxville, 

 in Canada, Albany, and New York, as well as different points in Mas- 

 sachusetts. The copper time-ball, held by a powerful electro-magnet 

 at the top of the mast on the Equitable Life Assurance Building, Bos- 

 ton, is released at noon by the clock at Cambridge. During 1879 ac- 

 cidents caused a small error in its fall on two days only, and on three 

 days it has been dropped at 12h. 5m. Os. 



The great equatorial of fifteen inches' aperture and the meridian 

 circle whose telescope has an aperture of eight inches have been kept 

 actively in use for the last three years. The former instrument has 

 been devoted almost entirely to photometric work. The problem of 

 astronomical photometry, roughly stated, is to determine the bright- 

 ness of all the heavenly bodies, so that all may be compared with 

 a single standard. Previous to the beginning of this work at the 

 Harvard Observatory, photometric measurements had been made al- 

 most entirely upon the planets and brighter stars, and there was no 

 definite knowledge of the amount of light emitted by the satellites and 

 fainter stars. At the outset of the work several hundred measure- 

 ments were taken of the brightness of the outer and inner satellites of 

 Mars, which measures have been taken accurately nowhere else. The 

 satellites of Jupiter and Saturn, including Hyperion, the faintest of 

 Saturn's satellites, were similarly measured. In addition to measuring 

 their brightness, a large number of determinations of the positions of 

 the satellites were made. A comparison was also begun of the light 

 of the sun and stars, with the idea of reducing all photometric mea- 

 surements to a common standard the light of the sun. This photo- 

 metric work has been continued until the light of all the known satel- 

 lites, except the two inner satellites of Uranus, has been measured. 



One of the most important series of equatorial observations has 

 been in connection with the eclipses of Jupiter's satellites. These 

 phenomena have proved exceedingly valuable as a means not only of 

 determining the orbits of the satellites themselves, but of measuring 

 the distance of the sun or the velocity of light, and of obtaining ter- 

 restrial longitudes. 



The observations of the mere appearance or disappearance of a sat- 

 ellite, however, can not be rendered sufficiently exact, and, to lessen 

 the errors, photometric observations have been made of the satellites 

 as they gradually enter or emerge from the shadow of Jupiter, using 

 the planet itself or another satellite as a standard. 



In order to furnish means for the comparison of the scales of stel- 

 lar magnitude, employed by different astronomers in their estimate of 

 the brightness of faint stars, a number of faint stars in the immediate 

 neighborhood of the north pole were selected for photometric mea- 



