Each tap of the observer's finger completed for an instant an electric circuit, and recorded a mark on the 'chronograph.' This is a large metal cylinder covered with paper, and turned by a carefully-regulated clock once in every two minutes. Once in every two seconds a similar mark was made by a current sent by means of the standard sidereal clock of the Observatory. The paper cover of the chronograph after an hour's work shows a spiral trace of little dots encircling it some thirty times. These dots are at regular intervals, about an inch apart, and are the marks made by the clock. Interspersed between them are certain other dots, in sets of ten; and these are the signals sent from the telescope by the transit observer. If, then, one of the clock dots and one of the observer's dots come exactly side by side, we know that the star was on one of the wires at a given precise second. If the observer's dot comes between two clock dots, it is easy, by measuring its distance from them with a divided scale, to tell the instant the star was on the wire to the tenth of a second, or even to a smaller fraction. Whilst, since the transit was taken over ten wires, and the distance of each wire from the centre of the field of view is known, we have practically ten separate observations, and the average of these will give a much better determination of the time of transit than a single one would. But let the watcher be ever so little too slow in setting his telescope, or ever so little late in placing himself at his eye-piece, and the star will have passed the wire, and as it smoothly, resistlessly moves on its inexorable way, will tell the tardy watcher in a language there is no mistaking, 'Lost moments can never be recalled.' The opportunity let slip, not until twenty-four hours have gone by will another chance come of observing that same star. It is the stars that are chiefly used in this determination, partly because the stars are so many, whilst there is but one sun. If, therefore, clouds cover the sun at the important moment of transit, the astronomer may well exclaim, so far as this observation is concerned, 'I have lost a day!' The chance will not be offered him again until the following noon. But if one star is lost by cloud, there are many others, and the chance is by no means utterly gone. Beside, the sun enables us to tell the time only at noon; the stars enable us to find it at various times throughout the entire night; indeed, throughout both day and night, since the brighter stars can be observed in a large telescope even during the day.