Topic of Book
Denny explores five technologies – bow and arrow, waterwheels and windmills, siege engines, clocks and the governor – with a particular focus of applying physics to understand how they work.
Important Quotes from Book
Watermill and Windmill
Our oldest man-made power source, waterwheels, was widespread across the Old World by the first century BCE.
It is hard to overstate the historical importance of waterwheels. Their development over the millennium from 500 CE to 1500 CE represents mankind’s most outstanding technological development of this period (Britannica). Waterwheels powered most of the Old World civilizations during this millennium and it is difficult to imagine what the world today would be like, had we lacked these machines during this formative period.
Waterwheel numbers increased substantially during the Middle Ages, when there was an acute shortage of labor, making laborsaving machines more cost-effective. It is widely considered (Britannica; Mason; Usher) that the most dramatic industrial consequences of waterwheels occurred in the Middle Ages, when the scale of milling increased considerably with the development of large towns.
The power of European waterwheels increased by a factor of three during the eighteenth century, to perhaps 10 kW. This improvement became possible because millers and millwrights of this century began to adopt a more systematic and scientific approach, characteristic of the age, to mill design and construction. Because of their importance, much effort went into the scientific investigation of waterwheel efficiency during this period.
It amazes me that such a significant nineteenth-century scientific and engineering effort went into improving an instrument first introduced nearly two thousand years earlier. This fact provides a clear illustration of the absolute indispensability of waterwheels to the development of European and New World commerce and industry.
Since undershot wheels worked in slow streams they could be erected in many geographical regions that lacked fast-flowing water, such as the flat plains upon which most classical civilizations developed. So undershot wheels proliferated, and remained the most common type of waterwheel from classical antiquity until overtaken by overshot wheels in the thirteenth century CE (Britannica). Even after the development of the overshot wheel, they remained popular right up until steam engines became efficient and widespread in the middle of the nineteenth century CE.
By 1820 France alone had 60,000 waterwheels… Real estate, in this case running water frontage, became ever more expensive. Water head was squabbled over, and every last foot was needed. Overshot wheels required a large head—the drop-in height must be about the paddle diameter, between 2m and 10m—and so were usually confined to hilly areas, or required extensive and expensive auxiliary construction, such as mill races that ran for hundreds of meters. Undershot wheels, on the other hand, could operate with less than 2m head and so could be located on small streams in flat areas, near population centers. Thus they remained important well beyond the period when scientific investigation had shown them to be relatively inefficient.
Waterwheels were of economic importance well into the twentieth century. They were then eclipsed by turbines.
There is no evidence for the use of windmills in classical Greece or Rome… a key feature of windmills is their relatively restricted geography.
The Islamic vertical windmill—resembling a revolving door (James), complete with cylindrical housing—was developed in windy Persia or Afghanistan in the ninth or tenth century CE. It was widespread in parts of the Middle East in the tenth century.
Windmills were a common feature of flat Holland and the north German plains by the twelfth century CE. At about this time, or shortly thereafter, windmills appeared in the flat fenlands of eastern England, and in northern France.
The earliest windmills in Europe were the post mills… The whole mill, sails and all, could be rotated about the post as wind direction changed. This was usually achieved by the miller turning a capstan wheel attached to winding gears. Here we have the main reason why windmill development was slower than that of waterwheels, and why wind power was limited geographically. Winds are more variable than water. Water can be channeled so that, by adjusting sluice gates, the headrace flow is maintained constant (except in drought conditions). The four winds, however, can gust and blow, or fall calm.
By the 1500s smock-mills appeared alongside post mills… This process was extended when the tower mills (or turret mills) appeared. These consisted of stone or brick bodies, with only the cap and sails rotating about a vertical shaft. The stronger, taller bodies of tower mills meant that even bigger sails could be attached. The diameter of sails was now limited (to about 90 feet) by the available lengths of pitch-pine timber spines (stocks) rather than by windmill height. These tower mills could power several sets of millstones, and by the seventeenth century they were the backbone of Dutch life. The tower mills are the type we usually first think of nowadays, since many of them have survived to the present day quite well—being younger and more strongly built. They represent the pinnacle of windmill development.
Apart from drainage, the Dutch milling applications extended from grinding flour and sawing wood to milling paper, pepper, and snuff., to dyeing, fulling, and tanning in the cloth industry. The importance of windmills may be judged by the fact that there were approximately 9,000 of them in Holland in the nineteenth century, about one mill per thousand of the population.
The range of usable wind speeds was limited, and in practice this meant, in Holland, that a windmill of the older type could operate for about 2,700 hours per year (say 7 hours per day) on average. Later, more efficient windmills could operate in winds exceeding about 4m s_1 and this made quite a difference. Because such light winds are very common in northern Europe, the Dutch windmill could now operate for an average of 4,400 hours per year (12 hours per day). So, a straightforward increase in efficiency permitted a vast increase in productivity. Also, the lower minimum wind speed meant that the later designs produced more useful power, in a given wind, than did the earlier machines. At the peak of their development, windmills could sustain 50 horsepower output (37 kW).
Pendulum Clock Anchor Escapement
Hourglasses were the industrial timekeepers of medieval Europe.
A crucial development in the history of mechanical timepieces was the “train of toothed gears.” To connect the weight axle with the clock face, the crown wheel and verge escapement (so called because of the shape) were invented. This crown escapement, for short, was so basic to mechanical timekeeping that it lasted 500 years, beginning around 1275 CE.
Fifty years later (in the mid-1800s) a crisis developed, a power shortage that might have derailed the industrial revolution. This crisis was averted by scientific investigation of how governors work, mathematically. The importance of the problem is analogous to the longitude problem; it was a technology bottleneck that held us back, economically and in other ways. Resolution of the problem catapulted governors into the engineering limelight and established control theory and cybernetics as disciplines in their own right.
The number of patents taken out per decade in England between the years 1630 and 1840 CE. Clearly something happened in the middle of the eighteenth century. Inventions took off. The historians’ consensus is that the single most significant invention was James Watt’s steam engine.
One explanation is that there was a “fashion for innovation” at that time, epitomized by a small group of open-minded people who were eager to apply the fruits of science to improve the lives of their fellow man (and to get rich doing it).
So the first significant consequence of the centrifugal governor was to assist the spread of steam engines to applications outside the traditional one of pumping water. In the 1780s, steam power rose in importance.
So, in the initial stages of the industrial revolution the new steam engines expanded their range of operation because they were small, could be positioned anywhere, and could operate at a steady speed. The Watt governor is responsible for this last characteristic.
Waterwheels were our prime movers for about two millennia. Windmills were influential for about one millennium. The mobility of oxen, and later horses,1 was needed to plow fields and draw wagons, but where fixed power sources succeed the waterwheel or windmill was better. These could pump water to drain marshes and mines, and to irrigate fields. They could mill flour and saw wood. The mechanical power sources did not tire, kick, or defecate. On the other hand the waterwheel had to be built within a few hundred yards
of running water, and the windmill required open land in a windy part of the world. Both machines became efficient and effective. For specific tasks they became essential…
Equally, only the waterwheel could have powered the first phase of the industrial revolution. Draft animals could play only a minor role in these cases… With the introduction of Watt’s improved engine the traditional prime movers became largely redundant. So, waterwheels and windmills were important economic factors from antiquity until they were replaced by steam power at the end of the eighteenth century.
Counterpoise siege engines changed the face of medieval Eurasian warfare, with long-lasting consequences for military architecture and organization. Mangonels and especially trebuchets became gigantic… These castles grew large in response. The shape, size, and constitution of castles evolved in parallel with the threat from counterpoise engines. The shear scale of these castles required new levels of logistical and organizational skills.
The anchor escapement, when attached to a pendulum, gave us accurate clocks on land. The impact on timekeeping followed immediately upon the marriage of these two simple devices… Accurate navigation by traditional methods also needed accurate timekeeping. The Longitude problem had held back European exploration, colonization, and trade.
Centrifugal governors regulated the power of James Watt’s steam engines. The resulting engine had a dependably constant speed, and this permitted its use in many specific applications (such as spinning yarn) where it previously had not been used…
When it comes to estimating the importance of the centrifugal governor it is hard to separate it from the throttle valve and the sun-and-planet gears. Together they enabled the steam engine to power the industrial world, and this is one of the most important technological advances in our history. The centrifugal governor itself flourished for perhaps only 90 or 100 years from the 1780s; thereafter, superior regulators displaced them.