212 REPORT— 1861. 



are provided with various screw adjustments as to position, range, &c., and 

 a slender spring beneath the lever, ensuring that it shall not be accidentally 

 moved by wind, or other cause, until acted on by the powerful grasp of the 

 magnets. 



This instrument was found to answer admirably well. It may be observed, 

 in passing, that it gives the means of exploding mines at almost any distance 

 through telegraphic Avires, and by any moderate contact-making power, and 

 may admit of valuable applications hereafter for the explosion, at a determi- 

 nate instant, of mines for purposes of warfare. 



It is obvious that a certain loss of time must occur at this contact-maker, 

 in reference to our experiments — that, in fact, the total time registered by 

 the chronograph at D is too great by the minute interval that elapses 

 between the arrival of the galvanic current in the coils at a and the dipping 

 of the poles/, /into the mercury-cups. With the same battery power at E 

 and conducting wires, this delai/ is practically constant. Its amount, how- 

 ever, required to be determined, and the time, when converted into distance, 

 added to the gross transit-rate previously ascertained. 



For this purpose the following little apparatus was employed. Its prin- 

 ciple, though not the precise details of its construction, is shown in 

 fig. 6, PI. IV. Upon a vertical steel spindle (s) revolving upon an agate step 

 at bottom, and in a polished brass collar at top, a cylindric barrel is placed, 

 of 1 inch diameter, having an escapement-wheel and anchor-escapement (i>) 

 at its lower end, all the parts being made as light as possible. Upon the 

 upper end of the spindle a circular disk of Bristol board (cardboard),/ of 

 12f inches diameter, is secured by a light screw collar (t) gripping the disk 

 firmly, so that it and the spindle must revolve together. Both the upper and 

 under surfaces of the card-disk, for an inch or two from the circumference, 

 towards the centre, were slightly rubbed with violin-player's hard rosin, and 

 the whole, resting upon its base B, placed so that the disk should rotate 

 horizontally. A fine elastic silk thread is wound a few turns round the 

 barrel, and passing over the sheave (r) sustains a weight (W), by the descent 

 of which, when required, rotation can be given to the disk, &c., the weight 

 itself being large in proportion to the inertia of the rotating parts. By suit- 

 able changes in the disposition of the parts of the contact-maker (chiefly in 

 getting the cast-iron arm d, fig. 2, out of the way), it was placed at C with 

 respect to the disk ; so that the lower poles of the electro-magnets (a, a) were 

 just above the upper surface of the card-disk, and the short end of the lever 

 armature (e) just below the same, the card running free in the small space 

 between, and the centre of the magnet-poles being exactly at a radius of 

 6 inches from the centx-e of the disk. Nearly at right angles on the disk to 

 this, the chronograph (D) was placed and firmly fixed : a fixed point (shown 

 in part only in the fig. g), formed of a bit of cylindrical mahogany, with its 

 lower end rosined, was so fixed as to be about y^th of an inch above the 

 upper surface of the disk. The lever (m) of the chronograph, divested of its 

 forked pole, and having a small rectangular rod of brass substituted, was so 

 adjusted that its sustaining spring beneath should press this brass terminal up 

 against the under surface of the disk atp, directly below the fixed point or 

 stop (ff), and bending the cardboard there, press its ujjper surface into con- 

 tact with the lower end of ^. 



Thus the weight W being free to descend, this arrangement atj9 acted as 

 a detent to keep the disk from moving ; but when the lever (w) was pressed 

 down to start the chronograph, the disk immediately became released, and 

 began to revolve by the action of the weight W. At E the contact-making 

 battery, or one of equal power, was placed, one of its poles being connected, 



