TRANSACTIONS OF THE SECTIONS. 21 



Let the numbers of the scale divisions increase in the same direction in all the scales. 



The formula for changes of total force is of the form 



Variation in parts of total force=cV+rfH. 



Let D be a fixed scale representing the variations of total force ; / the length of 

 unit of scale adopted. Make on the edge of the second sliding piece a scale E, such 



that one division=i. Make a fixed scale F on the other side of the sliding-piece, 

 c 



■whose division = 4-. Place the scales so that when the slide is closed the zero- 



d 

 points shall be in a line. Draw also an index n on the sliding-piece, corresponding 

 to zero on scale E, and let the numbers of the scales increase in the same direction as 

 before. 



To use the instrument : — 1st. Move the first slide until the mark m is opposite to 

 the scale reading of the horizontal force on C ; find on B the scale reading of the 

 vertical force, and opposite to it on A is the number representing the variation of 

 dip from an assumed zero. 2nd. Move the second slide until the index n points to 

 the horizontal force reading on F ; then on D, opposite to the vertical force scale 

 reading on E, is the variation of the total force in parts of the whole force. 



Astronomy, Meteors, Waves. 



Account of /he Astronomical Instruments in the Great Exhibition. 

 By Dr. Bateman. 



Description of an Apparatus for making Astronomical Observations by 

 means of Electro-Magnetism. By G. P. and R. F. Bond, of the Cambridge 

 United States Observatory. 



The apparatus exhibited to the Section is the same which has been in use at the 

 Harvard Observatory, Cambridge, U.S., and is the property of the United States Coast 

 Survey. It consists of an electric break-circuit clock, a galvanic battery of a single 

 Grove's cup, and the spring governor, by which uniform motion is given to the paper. 

 Two wires pass from the clock, one direct to the battery, and the other through the 

 break-circuit key used by the observer, and through the recording magnet back to the 

 battery. The length of the wire is of course immaterial. When the battery is in 

 connection, the circuit is broken by the pallet leaving the tooth of the wheel, and is 

 restored at the instant of the beat of the clock, which is in fact the sound produced 

 by the completion of the contact restoring the circuit, the passage of the current 

 being through the pallet and the escapement-wheel alone. With the exception of 

 the connecting wires, and the insulation of some parts, the clock is like those in 

 common use for astronomical purposes. 



Several forms have been proposed by different persons for interrupting, mecha- 

 nically, the galvanic circuit at intervals precisely equal. In the present instance the 

 clock is of the form proposed by Mr. Bond. Prof. Wheatstone, Prof. Mitchell, Dr. 

 Locke, Mr. Saxton and others, have contrived different modes of effecting this object ; 

 the former several years since, but for a purpose distinct from the present. 



The cylinder makes a single rotation in a minute ; the second marks, and the ob- 

 servations succeed each other in a continuous spiral. When a sheet is filled, and it 

 is taken from the cylinder, the second marks, and observations appear in parallel 

 columns, as in a table of double entry, the minutes and seconds being the two argu- 

 ments at the head and side of the sheet. 



The observer with the break-circuit key in his hand or at his side, at the instant 

 of the transit of a star over the wire of a telescope, touches the key with his finger. 

 The record is made at the same instant on the paper, which may be at any distance, 

 many hundred miles, if required, from the observer. It is a well-established fact, 

 that not only may observations be increased in number by this process, but that the 

 limits of error of each individual result are also narrowed. As far as comparison^ 



