Jan. 4, 1877] 



NATURE 



203 



scope used with the west equatorial in solar observations 

 powerfully disperses the rays of light, which are carried 

 twice through a train of prisms. In photographing the 

 sun a lens of long focus is used, the light being thrown 

 upon it by movable plane mirrors. This plan of Prof. 

 Winlock's was adopted by the astronomers who went out 

 under the U.S. Government to observe the Transit of 

 Venus in December, 1875. 



The photometer, or light measurer, made by ZoUner, 

 has been used for three years by Assistant Prof. C. S. 



Peirce. The design is the accurate measurement of the 

 magnitudes of all stars in Argelander's Uranometria be- 

 tween 40" and 50° north declination, deter.nining these 

 magnitudes on a scale of uniform ratios of light, so that 

 the probable error of one observation shall not exceed the 

 tenth of a single magnitude. The great object of this is, 

 that throughout Europe and the northern part of the 

 United States there will be constantly enough of accu- 

 rately determined stars near the zenith to serve as com- 

 parisons for any star visible to the naked eye whose mag- 



FiG. 5. —Cambridge Star Spectroscope. 



nitude is to be estimated. The secondary object is the 

 prosecution of inquirirs with regard to the distribution of 

 the stars in space, their maguitudes and variability. 



The true time is daily given from this observatory to 

 the State-house and other places in Boston, and by means 

 of the telegraph lines to the whole of the New England 

 States. It is received directly at noon each day without 

 the intervention of any operator ; the various lines being 

 merely switched into the time line, the same click is heard 

 at the same moment over the Eastern States. 



Much more, however, than this is done for securing 

 accuracy of time at any hour of the day. If anyone 

 wishes to learn not only what the true time is, but whether 

 his own watch is a good timekeeper, he may readily do 

 so by a visit to the State-house in Boston. The arrange- 

 ment for this, introduced by Prof. Winlock, is as 

 follows : — The observatory clock is put in circuit at 

 one end of a telegraph line, connected with which, 

 at the State-house and other points, is an ordinary 

 telegraph sounder. When the clock breaks the circuit 

 by every second swing of the pendulum, a click of the 

 armature of the sounder is heard at each of these points. 

 The clock being so arranged that at every fifty-eighth 

 second the break ceases, and at every even five minutes 

 twelve breaks cease (no clicks being then heard), any 

 person can, by listening to the sounder, compare his own 

 watch with the standard clock. He can tell whether his 

 watch is fast or slow by watching when the sounder 

 ceases, the first click after the short pause being always 

 the beginning of the minute, and the first click after the 

 long pause the beginning of an even five minutes, as 

 shown by the face of the clock in the distant observatory. 



This standard motor clock is of course regulated with 

 extreme care. It is customary, for the government of its 

 rate of motion, to use shot of different sizes, which, ac- 

 cording to the size, produce a change in the rate of the 

 pendulum varying between 0*05 and o"io of a second per 

 day. These are used as the astronomical correction for 

 clock error may require. The time given by the standard 

 clock thus regulated is that of the meridian near the 

 State-house, sixteen seconds east of the observatory. Prof. 

 Winlock considers that the use of the telegraph sounder 

 gives a more satisfactory accuracy of time than can be 

 given by other clocks which are put within the circuit and 



controlled, as is usual, by the standard clock ; for in their 

 case a variation in the strength of the electric current 

 introduces an error in the beats of the pendulum, but the 

 telegraph sounder must give the time with entire accuracy. 

 With so much before one at Cambridge of which inte- 

 resting note could be made, one can do no more than 

 attempt to trace its early and munificent endowment, its 



Fig. 6 — Cambridge Spectroscope. 



earliest discoveries under its first labourers, and the steady 

 and recently very rapid advances not only in the highest 

 objects of an observatory — exactness throughout extended 

 series of observations — but in the exercise of professional 

 skill in the invention and manufacture of the best appli- 

 ances of the day for carrying on these investigations. 



