to Practical Astronomy. 151 



was anxious to devise some means by which it could also be 

 adapted to the shifting observatories used in the geodetic 

 survey- Here, large clocks being of course out of the ques- 

 tion, the great difficulty was the mechanical arrangement by 

 which the "clock contact" could be made with an ordinary 

 marine chronometer — a difficulty so great that, so far as I 

 am aware, it has not been previously overcome. After several 

 experiments I was enabled to plan an appliance, which has 

 been admirably constructed by a watchmaker at Williams- 

 town, and is found to work perfectly, without interfering 

 with the rate of the chronometer. It consists of an extra 

 wheel, of 60 teeth, which is in the seconds' arbor, the teeth 

 conical, with a slight rake, and, as in the astronomical clock, 

 one tooth is cut out as a minute data. The great delicacy 

 required in adapting contact springs, without producing un- 

 due friction, offered the chief obstacle ; but by placing the 

 opposing surfaces of the teeth and pallet at the most favorable 

 angle, and arranging so that the slightest lift of the pallet 

 broke contact, instead of making it, the principal difficulty 

 was overcome. The contact here is the reverse of that in the 

 clock ; for when the pallet is in the angle between two teeth, 

 the spring batiks on a platina stud, and the contact is com- 

 plete ; but each passing tooth lifts it sufficiently to break 

 contact for an instant. This peculiarity would, of course, 

 cause a registering style to make a long stroke and a short 

 space ; but the same effect as in the " clock contact" can be 

 attained by a modified relay, or reversed lever in the re- 

 gister. 



The success of this appliance to a chronometer at once ad- 

 mits of a perfectly portable chronograph being arranged, and 

 1 have had a register constructed to complete the apparatus. 



The fillet-paper used in the Observatory, although offering 

 several advantages, Avould be very inconvenient for a portable 

 chronograph ; the plan of the register, therefore, is different 

 altogether to the one in the Observatory. 



A metal drum, six inches in diameter, and eight in length, 

 covered with fine cloth, is made to revolve on a horizontal 

 axis in about 35 seconds, by means of a weight and clock- 

 work, regulated by a conical pendulum. A small waggon or 

 carriage is made to traverse smoothly and easily on a rail- 

 way, in a direction parallel to the axis of the drum : this car- 

 riage carries the electro-magnets, levers, and styles, which 

 arc so adjusted that when the electro-magnets are brought 

 into action, the styles will impinge on the drum. The move- 



