100 YEARS AGO

The surface of the globe has not always been as we now see it. When, in the past, the surface had a temperature of about 400°F., what is now the water of the ocean must have existed as water vapour in the atmosphere, which would thereby — as well as because of the presence of other substances — be increased in density and volume. Life, as we know it, could not then exist. Again, science foresees a time when low temperatures, like those produced by Prof. Dewar at the Royal Institution, will prevail over the face of the earth. The hydrosphere and atmosphere will then have disappeared within the rocky crust, or the waters of the ocean will have become solid rock, and over their surface will roll an ocean of liquid air about forty feet in depth. Life, as we know it, unless it undergoes suitable secular modifications, will be extinct. Somewhere between these two indefinite points of time in the evolution of our planet it is our privilege to live, to investigate, and to speculate concerning the antecedent and future conditions of things.

From Nature 28 September 1899

50 YEARS AGO

Except for the input and output mechanism, the EDSAC [electronic delay storage automatic calculator] has no moving parts, all computing and control operations being performed by means of electronic circuits. Within the machine numbers are expressed in the scale of two and are represented by trains of pulses synchronized with a continuously running ‘clock pulse’ generator. … Numbers expressed in this form are stored by a method depending on the use of an ultrasonic delay unit. The pulses are applied to a quartz crystal mounted at one end of a column of mercury, and give rise to ultrasonic pulses which travel through the mercury with the velocity of sound. On arrival at the far end they strike a second quartz crystal and are reconverted into electrical pulses. The time taken to traverse a column of mercury 5 ft. long is about 1 msec., and the interval between the beginning of one pulse and the beginning of the next is 2 msec.; there can thus be as many as 500 pulses passing down the column at any one time. On emerging from the delay unit the pulses are amplified and reshaped, and passed back to the input of the delay unit. They then continue to circulate indefinitely and are available when required.

From Nature 1 October 1949.