The announcement comes from the electrician's testing-room that the continuity is perfect, and with this assurance the engineers go on more boldly with the work. In point of fact the engineers may be said to be very much under the control of the electricians during paying out; for if the latter report anything wrong with the cable, the engineers are brought to a stand until they are allowed to go on with their operations by the announcement of the electricians that the insulation is perfect and the continuity all right. The testing-room is where the subtle current which flows along the conductor is generated, and where the mysterious apparatus by which electricity is weighed and measured--as a marketable commodity--is fitted up. The system of testing and of transmitting and receiving signals through the cable from ship to ship during the process of paying out must now be briefly referred to. It consists of an exchange of currents sent alternately every ten minutes by each ship. These not only serve to give an accurate test of the continuity and insulation of the conducting-wire from end to end, but also to give certain signals which it is desirable to send for information purposes. For instance, every ten miles of cable paid out is signalized from ship to ship, as also the approach to land or momentary stoppage for splicing, shifting to a fresh coil, etc. The current in its passage is made to pass through an electromagnetometer,[34] an instrument invented by Mr. Whitehouse. It is also conveyed in its passage at each end of the cable through the reflecting-galvanometer and speaking-instrument just invented by Professor Thomson; and it is this latter which is so invaluable, not only for the interchange of signals, but also for testing purposes. The deflections read on the galvanometer, as also the degree of charge and discharge indicated by the magnetometer, are carefully recorded. Thus, if a defect of continuity or insulation occurs it is brought to light by comparison with those received before. For the first three hours the ships proceeded very slowly, paying out a great quantity of slack, but after the expiration of this time the speed of the Agamemnon was increased to about five knots, the cable going at about six, without indicating more than a few hundred pounds of strain upon the dynamometer.