3 86 



SCIENCE. 



be pasted upon the slide, instead of the disk of varnish, 

 to serve as a dark background. A cell ring is then ap- 

 plied around the edge of the disk, and the object 'is fast- 

 ened in the centre of the cell by means of mucilage or 

 glue. This done, the cover is placed upon the ring and 

 cemented down as described above. — {Compendium of 

 Microscopical Technology.) 



THE DEARBORN OBSERVATORY. 



The annual report of the Board of Directors of the 

 Chicago Astronomical Society, together with the report 

 of the Director of the Dearborn Observatory, dated May, 

 1 88 1, is now published. 



The first report is brief, and states that the Society has 

 entered into a contract with the city of Chicago for fur- 

 nishing standard time to the City Hall. In order that 

 this contract may be more satisfactorily fulfilled, the 

 Directors of the Society have ordered from Messrs. 

 Howard & Co., of Boston, two new clocks, which will 

 cost about $iodo. The cost of running wires and other 

 equipments has been $574- 



The friends of the Society have contributed the funds 

 required to meet the immediate wants of the Observatory, 

 but the Society reiterate the often-repeated call for a per- 

 manent endowment, which will not only enable it to con- 

 tinue its present course of action, but to enlarge its 

 sphere of astronomical work and take an honorable place 

 among the prominent astronomical Observatories of 

 Europe and America. The Directors express a hope 

 that the time has arrived when the public-spirited citi- 

 zens of Chicago will contribute the amount to accom- 

 plish this object, and we heartily trust that the confi- 

 dence expressed in this respect may receive a prompt 

 confirmation. The Dearborn Observatory is built and 

 equipped with one of the finest equatorials in the United 

 States ; the question of endowment is therefore one which 

 calls for immediate action. 



In the second report Professsr G. H. Hough states the 

 nature and amount of the astronomical work carried on 

 at the Dearborn Observatory during the past year by 

 himself and Messrs. Elias Colbert and S. W. Burnham. 



The planet Jupiter has received the attention of a large 

 number of astronomers during the past two years, espe- 

 cially of amateurs, and much writing of a miscellaneous 

 character has appeared on the subject. As the proper 

 study of markings and spots on celestial objects require 

 the use of a telescope of great optical power, combined 

 with good definition, the following report of observations 

 on the planet Jupiter, made with the Dearborn equa- 

 torial, which possesses these conditions, will be read with 

 interest, especially as they do not confirm many observa- 

 tions made under less favorable circumstances. 



The planet Jupiter was made a special study during 

 the past year. The first observation was secured on May 

 6, 1880, and the last on January 30, 1 881 . During this 

 period the various spots and markings on his disc were 

 subjected to micrometer measurements whenever prac- 

 ticable. It is readily apparent to any one who has exam- 

 ined contemporaneous drawings or sketches made by 

 different observers and telescopes, that they are gener- 

 ally unreliable, unless based on micrometer measurement, 

 and frequently give rise to erroneous deductions with 

 regard to the phenomena in question. We believe the 

 time has passed when mere estimations or sketches are 

 of value in any department of practical Astronomy. 

 Jupiter presents such a variety of phenomena on his disc, 

 at different times, that it has been accepted as an estab- 

 lished fact that his surface is subject to sudden and rapid 

 changes, which may be accomplished in a few days or 

 even a few hours. 



The observations made at the Dearborn Observatory 

 during the past two years does not confirm this statement. 

 On the contrary, all minor changes in the markings or 



spots have been slow and gradual, such as might be pro- 

 duced by the operation of measurable mechanical forces. 

 In fact, the principal features have been permanent, no 

 material change being detected by micrometer measure- 

 ment. 



The following is a summary of the observations on 

 Jupiter : 



GREAT RED SPOT. 



Longitude, 37 nights 560 measures. 



Latitude, 12 " 34 " 



Length, 20 " 67 " 



Breadth, 10 " 32 " 



Position of maj. axis, 5 " ft 16 " 



Total 709 " 



EQUATORIAL BELT. 



Observed on 26 nights — 



Position of the North Edge 87 measures. 



Latitude " " 34 " 



Width of the Belt 53 



Total 174 " 



EQUATORIAL WHITE SPOTS. 



Observed on 18 nights — 



Longitude 240 measures. 



Latitude 15 " 



Total 255 



POLAR SPOTS. 



Observed on :2 nights — 



Longitude 144 measures. 



Laiitude 40 " 



Total 184 " 



Being a total of 1,379 micrometer measurements. 



From the micrometer measurements for longitude of 

 spots, the equatorial diameter of the planet is deduced on 

 50 different nights, and from the latitude measures, the 

 polar diameter on 13 nights. 



The following deductions have been drawn from these 

 observations. 



ROTATION OF JUPITER. 



The period of the planet's rotation, as obtained by dif- 

 ferent observers, has varied between o h 49" 1 and 9' 1 56"'. 

 The observations made on the great red spot during the 

 opposition of 1879, gave lor the rotation period about 

 9 1 ' 55'" 34" ! being 8 seconds greater than the previously 

 accepted value. 



The discussion of our longitude measures on the great 

 red spot, made from September 25, 1879, to January 27, 

 1 88 1, comprising a period of 490 days, gives for the mean 

 value 9 h 55" 1 35.2 s , 



When the individual observations are compared, how- 

 ever, with this value, there is found to be a well marked 

 maximum displacement of the center of the spot amount- 

 ing to 1 ".4 of arc, indicating that the center gradually 

 oscillated to this extent in longitude, corresponding to 

 an actual displacement on the surface of Jupiter 3,200 

 miles. 



The observations are all well represented by making 

 the rotation period depend on some function of the 

 time. 



The period 9 U 55'" 33.2 s + o 18 s ,// satisfies all the 

 observations with a mean maximum error of o".5 of arc. 

 In which the zero epoch is September 25, 1879, an d t is 

 the number of days after that date. 



This formula gives for the rotation at the date January 

 27, 1 88 1 , 9'' 55"' 37.2 s , agreeing essentially with the value 

 deduced directly from the observations made during the 

 two months previous to that date. 



The rotation period derived from the observation of 

 polar spots was as follows ; 



