August 17, 1882] 



NATURE 



369 



out affecting its rate. The barrel can be taken out of its 

 Ys if desired, or one end of the barrel can be lifted by a 

 small lever, so that it can be turned around to put on or 

 take off a fresh sheet of paper. In practice several sheets 

 of paper are put on at a time, so that the last one has 

 simply to be removed when it is filled, and the pen- 

 carriage moved back (to the right) to continue the record. 

 This can be done without stopping the chronograph. 



A second of time is C36 inches in length, in the usual 

 adjustment. The governor is a double conical pendulum, 

 acted on directly by the weight. It thus tends always to 

 run too fa-t , as it runs faster and faster, the pendulum 

 bobs fly out, and finally strike the point of a horizontal 

 hook shown in the drawing. This hook is attached to a 

 little cylinder of brass embracing the vertical axis (also 

 shown), and when the hook is touched by the pendulum 

 bob (as it is shown in the cut), the hook and the brass 

 cylinder are carried about the axis through a certain 

 angle. The work thus done diminishes the speed of the 

 pendulum, which falls in towards the axis slightly. In 

 this way the governor and also the barrel rotate alter- 



nately a little faster and a little slower than the desired 

 mean rate, but these variations are vanishingly small and 

 of no account whatever in astronomical work. The 

 pendulums strike the hook on the average from sixty to 

 ninety times per minute. 



The pen-carriage is nothing but an electromagnet 

 mounted on a frame, which is driven by an endless screw 

 from right to left in the cut. The carriage may also be 

 lifted by the hand and moved in either direction. This 

 is a great convenience in certain kinds of work, such as 

 comparing a number of clocks together. The record for 

 each clock can be separated from that of every other 

 clock by a blank space. 



The pen is of glass, filled with a thick ink made accord- 

 ing to the following formula which is used at the Naval 

 Observatory. This ink does not freeze in winter weather. 



Water 4 fluid ounces. 



Alcohol 2 ,, 



Concentrated glycerine 1 fluid drachm. 



Crystallised Aniline Blue 40 grains. 



Filter very thoroughly and draw off for use through a stop- 



cock. A common stylo-graphic pen, if held nearly 

 vertical and weighted with a little piece of lead, is nearly 

 as good as the glass pen, and somewhat cleaner. 



The signals from the clock and observing key are 

 received through the two flexible wires shown in the cut. 

 These signals can be repeated, by connections to screw- 

 posts, on the pen- carriage. 



The whole machine is light and portable. It ta'tes, 

 say, fifteen minutes to move it from one room to another. 

 It can be worked equally well with a break or a make- 

 circuit. Its price is 32500 dollars. The makers of our 

 chronograph are Fauth and Co., Washington, D.C., but 

 the design is that adopted by the Clarks. 



The first double conical pendulum of this kind was 

 made bv Dr. Henry Draper, and applied to the driving- 

 clock of his photographic telescopes. The first governor 

 on this principle was adopted by Alvan Clark and Sons, 

 for driving the heliostats used in the United States 

 Transit of Venus Expedition of 1874. These governors 

 had, however, only a single pendulum, and not two 



crossed pendulums, as in the cut. I am induced to send 

 you this brief account of a simple and useful device 

 which has had a thorough trial of 31 years (it was exhibited 

 by G. P. Bond at the Crystal Palace in 1851), which is 

 always satisfactory ; which never gets out of order ; 

 because it is a standing wonder to us, on this side of the 

 water, why the expensive and complicated double-pen 

 chronographs continue to be made and used in England, 

 and on the Continent. The inclosed sheet, which is 

 selected absolutely at random from a pile of such records, 

 will show the kind of work these machines will do ; and 

 all the questions which have been agitated with regard to 

 the relative accuracy of one and two pen-chronographs, 

 seem to me to have been practically settled by the obser- 

 vations made at our principal observatories for a score 

 of years past. I need only mention the longitude cam- 

 paigns of our Coast Survey, of the Naval Observatory, of 

 the Army Engineers, and the standard work of the 

 Transit circles of Washington and Harvard College, in 

 this connection. 



