Step 2 of my strategic plan: Post my favourite class assignments from the past year, which will start this week with manure and will eventually end with sewage. Enjoy.

We tend to think of nineteenth century cities like Pittsburgh as industrializing under the power of steam. But Joel Tarr argues that an older technology also drove the development of the great cities of the steam age.

In 1775 James Watt patented the steam engine, a machine that would become a symbol of the industrial revolution. Forty years later, Benjamin Latrobe opened a steamboat engine workshop on the banks of the Monongahela River in Pittsburgh. The power source that Latrobe used to build his engines? Two blind horses.

Horses like Latrobe’s were a central cog in the nineteenth century urban economy. They were hooked up to engines through circular sweeps, rotating platforms and treadmills, and harnessed to vehicles on wheels and tracks. City horses hauled steel, powered ferries, pressed bricks. They were the source of valuable manure and even more valuable carcasses. They were the catalysts for the paving of streets and the suburbanization of cities.

But, like the combustion engine, their great success as a technology also contributed to their eventual decline. Thanks to the work of Joel Tarr, a professor of history and policy at Carnegie Mellon University, and his colleague Clay McShane, a professor of history at Northeastern University, we are rediscovering how horses hauled cities like Pittsburgh into the modern age.

Horses at construction site

Horses at Cincinnati subway construction site, probably the 1920s. Image credit: University of Cincinnati Library.

Rediscovering the horse as an urban technology

Joel Tarr thought he was done with horse manure back in 1971. The Jersey City native had joined the faculty at Carnegie Mellon in 1967 with a background in urban political history and an interest in how the modern city had been shaped by transport technologies. During his research, he kept coming across historical complaints that he thought might be interesting to a general audience.

The article he submitted to the magazine American Heritage was a vivid account of the problems faced by a horse-driven city, including the staggering scale of manure logistics:

…as health officials in Rochester, New York, calculated in 1900, the fifteen thousand horses in that city produced enough manure in a year to make a pile 175 feet high covering an acre of ground and breeding sixteen billion flies, each one a potential spreader of germs.

“Urban Pollution— Many Long Years Ago”, American Heritage, 1971

Tarr noted that the eventual solution to these problems was the adoption of a new technology, one that harnessed machines rather than animals. “Apparently the editorial board got into a big fight about it because some of them thought that it was an apology for the automobile,” he recalls.

Controversial from the start, the article prompted several newspaper editorials and is still cited today in debates about pollution. But after creating a stir, Tarr moved on, reasoning that “someone else could worry about the horse manure.” And so he pursued his interests in the environmental and technological history of cities and left the subject of horses alone for more than twenty years.

But while the details of the manure problem lived on in the public imagination, Tarr knew that there was much more to say about the importance of horses in the history of our cities. In 1995, he refused to allow the manure article to be reprinted in an anthology and instead asked if he could write a new article on the topic with his friend McShane, who had recently published a history of cars in cities. From that first article, the project ballooned out into a decade of scholarship and co-authorship of their 2007 book on horses as an urban technology, The Horse in the City: Living Machines in the Nineteenth CenturyThe book is a detailed examination of the centrality of horses to cities, focusing on New York City, Boston, and Tarr’s adoptive home, Pittsburgh.

From steam power to horse power

Tarr and McShane emphasize that the horse was viewed as a “living machine,” valued primarily for its ability to provide power. As machines, horses were an integral part of the economy, even after the advent of the steam engine.

After refining the steam engine, James Watt invented a standard measure of mechanical work – 33,000 foot-pounds of work per minute, or 1 horsepower. This unit allowed customers to estimate how many horses an engine could replace and to gauge whether replacing their horses would be economic. In many cases, it wasn’t. For much of the nineteenth century, horses were the engine of choice for applications that required flexibility or mobility and for businesses that could not afford a large capital outlay.

But the one application in which horses were irreplaceable was ground transport within the city. Goods from the expanded railway and steamboat lines could only be distributed to their final destinations under the power of horses, which meant that horse-drawn transport grew more efficient in parallel with steam technology. Innovations in breeding produced larger and larger horses in the pursuit of (as one agricultural reformer put it) “the best machine for turning food into money.” These industrial-strength horses could pull even larger loads after the development of lighter vehicles made with modern materials.

One resident of Pittsburgh remembered the “pandemonium of noises” produced by horse transport in the 1860s:

Numerous wagons, hauling heavy pigs of iron and iron products, timber wheels with anywhere from six to fourteen horses from which huge and unwieldy vehicles hung castings of many tons’ weight, the clattering omnibus, the rattle of the mail wagons, drays […] and other conveyances common to traffic.

George Thornton Fleming,1904

This was the cacophony of Pittsburgh’s developing steel industry, the sound of a modern city propelled by coal and hooves.

Photo of 1897 traffic

Horse-drawn wagons and carriages, an electric trolley car, and pedestrians congest a cobblestone Philadelphia street in 1897. Image credit: National Archives and Records Administration, 30-N-36713.

Shaping the city

The structure of Pittsburgh’s neighborhoods today still reflects the age of horse-drawn vehicles. Public transport began with road vehicles called omnibuses, but gathered momentum with one of the most influential urban innovations of the nineteenth century, railed “horsecar” lines. These tracks, the precursors of the cable car and electric streetcar systems, provided a smooth ride that omnibuses could never achieve on cobbled pavements that were optimized for horseshoe traction. The benefits of the tracks were not just to the spines of riders, but to the speed the horses could travel, the number of riders they could haul, and the amount of profit their owners could make.

The first lines were laid in 1863 and by 1890 the average Pittsburgh resident took 192 horsecar trips per year. The tracks had grown along the lines of least resistance, following valleys and avoiding the worst of Pittsburgh’s steep hills. As the lines expanded, ridership increased at a rate much faster than population growth, reflecting Pittsburgh’s shift to the suburbs; Residents could now live further away from downtown and make a daily commute to work. Wards within an hour’s smooth ride of downtown were suddenly more desirable than when they were a longer, more expensive and more bone-jarring omnibus ride. The relatively flat Eastside saw the biggest growth – between 1870 and 1890 it grew from 5,350 dwellings to 17,604. Construction boomed in areas within a ten-minute walk of a horsecar line. Tarr and McShane write that “the greater speeds allowed Americans to fulfill the new dream of the middle class, a detached home with a yard on the outskirts of a city.” Meanwhile, downtown was losing residents to the new suburbs and slowly transforming into a true central business district.

Tarr and McShane point out that the horsecar alone did not cause these changes in Pittsburgh and other growing cities. Factors like economic expansion, population growth and a new appreciation for suburban life played an important role, but the horsecar was the technology that allowed these trends to play out, and it set the patterns that were extended in the twentieth century by the streetcar and the gas-fueled automobile.

Problems with the living machine

In 1872, American horses came down with a terrible case of the flu. Several Northeastern cities ground to a dramatic halt. The horse flu epidemic cut off city supplies, grounded fire departments, and isolated suburbs from their vital horsecar lines. When one commentator later warned that another epidemic would reduce New York City to “straits of distress,” he concluded that although “cities have been made by building around the horse there is no necessity for keeping him permanently as their centre.” As the century progressed, more and more objections were made to the city’s dependence on horses.

Like all technologies, horses had their downsides. They were living creatures, susceptible to disease, unreliability, and even personality. They required an enormous infrastructure of foul-smelling stables, with stockpiles of hay that posed a significant fire hazard. But above all, horses were prolific polluters. The average city horse unleashed 25-35 pounds of manure and two to three gallons of urine per day.

Horse manure started out as just one of the many hazards of urban life, but as the century progressed, the exploding city horse population became a source of public angst and newspaper editorials. To make matters worse, by the 1880s the bottom had fallen out of the manure market.

Fresh manure had long been a valued commodity, sold by stable owners and street sweepers to farmers on the urban periphery. But thanks partly to competition from new guano and rock phosphate fertilizers, the price of manure had fallen to less than a quarter of its worth. A New York Times editorial from 1881 conveys the confusion caused by a city decision to declare summer dumping grounds off-limits amidst the glut of manure: “Public health nuisance: No place for stable manure—What is to become of it?” By 1908, one journalist claimed that 20,000 New Yorkers died each year from “maladies that fly in the dust, created mainly by horse manure.” The biggest problem was that the accumulating piles were a favorite breeding ground for flies, a vector for life-threatening diseases like typhoid.

Part of the solution to the manure problem was technological. By 1902 most horsecar lines had transitioned to electric trolleys only a decade after they had been first introduced. But the manure problem itself was not necessarily responsible for the speed of this change. Tarr and McShane argue that in many cases, the new technology was rapidly embraced by horsecar companies because these companies did a tidy side business in land speculation. Horsecar lines had the reliable effect of pushing up property prices wherever they were laid, but by the late 1880s, horsecar lines had mostly expanded as far as they could within a one-hour commute of downdown. With the increased speed of electrified trolleys however, horsecar companies could expect to double that radius and reap the rewards in real estate. As a result, these companies became intimately involved in urban politics and in many cases bought themselves influence on city councils to ensure they received the necessary franchises. Within a decade, most of the lines had switched over to electric.

For a few more decades, horses were still favored for tasks like fighting fire, hauling waste, and making neighborhood deliveries. But by the end of World War II, even these jobs fell to the automobile. The horse manure problem was solved and the age of the car had begun.

The technological solution

Despite the initial optimism that cars were a clean and efficient alternative to the horse, the new technology has also become a victim of its own success. The burning of fossil fuels generates air pollution that can be as hazardous to human health as the diseases spread by flies, and it releases carbon dioxide that contributes to climate change. A century after the decline of the horse, we are again facing a chronic pollution problem.

Embedded among the engineers and policy faculty of Carnegie Mellon, Tarr has consistently pursued historical questions that provide perspective for contemporary policy debates, particularly the problems of urban waste. But ever since Tarr published that first article on the horse manure problem, commentators have repeatedly used the story as a parable about the wonders of technological fixes to environmental problems. For instance, Steven Levitt and Stephen Dubner used the story of the horse in their 2009 book SuperFreakonomics to justify the use of radical geoengineering solutions to climate change.

Tarr himself doesn’t believe technological change is always a panacea. “Why do we automatically assume that every new device will be better?” he asks. He has made urban technological change one of his specialties because he believes it is important that we understand the drivers of change. “History circles,” he explains.

This particular circle has come around quickly. In Tarr’s office there is a reproduction of a magazine photo hanging prominently amongst the accumulated books and papers; in it stands his father in a worker’s cap, cigarette between his lips, at work under the harsh light of the night shift at a shipyard. He had been one of those workers who built the urban landscape with the help of a living machine. “He had a horse,” Tarr says, “back when he was in the scrap business in New York. He had a horse called Shivers, and that’s just about all I know about it.”

Where to Find Out More

The Horse in the City: Living Machines in the 19th Century by Clay McShane and Joel Tarr Johns Hopkins University Press, 2007.

“The Centrality of the Horse in the Nineteenth-Century American City,” Clay McShane and Joel Tarr, In Raymond A. Mohl (ed.), The Making of Urban America Scholarly Resources, Inc., 1997.

“The Horse Era in Pittsburgh,” Joel Tarr, Western Pennsylvania History, Summer 2009, 28-41


Carey Morewedge serves up an imaginary feast

Dear Blobologist,

I know I’ve been neglecting you recently. As a peace offering, here’s a class assignment I did a few weeks ago, based on an interview with a researcher from my university. I hope it will tide you over.


Cristy xx


Imagine your favorite food.

Chocolate, cheddar cheese, chicken tikka masala, whatever your weakness, picture it just out of reach, glistening enticingly.

Although this food isn’t real, your body might be responding as if it were. Perhaps your mouth is watering or maybe you’re feeling the pang of cravings. But Carey Morewedge, a psychologist at Carnegie Mellon University, says food fantasies can have an even stranger effect; he has shown that we can be satisfied by imaginary food.

When Morewedge and his collaborators, Young Eun Huh and Joachim Vosgerau, began their research into imaginary feasts, most psychologists believed that the more you thought about food, the more you craved it. The problem with this idea was that when you eat food in real life, you crave it less rather than more. Our first mouthful of a favorite dish makes us desire it more, but as we eat bite after bite, we start to lose interest. This loss of interest is called “habituation” and is part of every pleasurable experience, from food to sex to watching Gangnam Style.

So why doesn’t imagining food also make us lose interest? Morewedge and his group asked this question because when we imagine an experience, our bodies and minds often respond as if that imagined experience were real. They guessed that the reason previous studies had not observed habituation was because study participants didn’t take their imaginary experiences far enough.

“If I’m thinking about a Chipotle burrito, I’m thinking about the shape, what’s inside it, what it will taste like on the first bite, what it might smell like, or how warm it might be,” says Morewedge. “But I do not think about biting, chewing and swallowing the whole burrito.”

So the researchers asked people to think about biting, chewing and swallowing M&Ms. Each person in their study imagined performing 33 repetitive actions: either inserting 33 quarters into a laundry machine, inserting 30 quarters into the machine and then eating 3 M&Ms, or inserting 3 quarters into the machine and eating 30 M&Ms. Inserting quarters was chosen as a control for imagining an action similar to picking up candy.

After their mental exertions, the participants were allowed to eat as many M&Ms as they wanted during preparation for a fictitious “taste test.” Psychologists often include these kinds of deceptive scenarios to prevent people guessing what behavior is being measured, which can influence their response.

After each experiment, Morewedge’s team weighed the M&M bowl to see how much the participant had eaten. The results showed that people who had imagined eating 30 M&Ms ate almost half as many real M&Ms as those who imagined eating only three. In effect, they had satisfied their desire for M&Ms without actually eating any.

This only worked when people pictured eating the M&Ms. When they instead imagined moving the candy into a bowl, the people who moved more imaginary M&Ms ended up eating more real M&Ms, rather than fewer. That kind of imagery only whetted their appetites.

But was this really habituation? To test this, the researchers looked for one of the hallmarks of habituation, called sensory specificity or the “dessert stomach” effect.

“We’ve all heard of this phenomenon,” says Morewedge. “When you go to a restaurant, you finish your entree and you can’t even imagine eating another bite. And then someone rolls out a cart of cheese or cakes, and all of a sudden you have a renewed interest in food.”

Just like real habituation, the imaginary M&Ms did not affect participants’ desire for other types of food, in this case, cubes of cheddar cheese. It worked the other way as well: eating more imaginary cheddar cheese meant people tended to eat fewer real cheese cubes, but it had no effect on how many M&Ms they ate.

So does this mean you can think yourself thin? Probably not. The dessert stomach effect is one reason why Morewedge doesn’t think we’ll see their results become the next diet craze. Imaginary eating habituates you to the food you have imagined, but makes other foods seem even more appealing.

Instead of using the research to come up with a diet miracle, the group is trying to apply their results to other contexts, like cigarette smoking, to see if mental habituation might be a useful tool for modifying addictive behavior.

But even if we are never able to harness the power of that imaginary chicken tikka masala for practical use, Morewedge and his colleagues have made an important theoretical advance. The line between imagination and physical experience is blurrier that we used to believe — a pleasant idea to contemplate the next time you get a hankering for something just out of your reach.

m&ms on asphalt

Photo credit: Flickr user Zen. Shared under this Creative Commons license

New York City: Indianapolis of the East

New York city. It’s about ten times bigger than Indianapolis, and about a bazillion times more famous. But to physicists Luis Bettencourt and Geoffrey West, NYC is just Indianapolis with seven million extra people. That’s because hidden beneath the apparent diversity of different cities, they’ve found certain patterns showing up again and again. These bizarrely uniform patterns suggest that, given enough people, Indianapolis would have the cultural and economic clout of NYC. But they also suggest that we have been making a fundamental mistake every time we try to understand what makes a city healthier, wealthier, or better looking than its rivals.

Times Square by Vikram Vetrivel via Flickr

Indy 500 Race Day by momentcaptured1 via Flickr

The patterns Bettencourt, West and their colleagues uncovered relate to population size. If you told them the population of any city in the US, without knowing anything else about it, they could give you a pretty accurate prediction of its vital statistics: average income, infrastructure, crime rate, disease prevalence. That’s because most of these figures change regularly with population size in one of two ways:

The bigger the city, the more efficient (by 15%)

The more people that live in a city, the more infrastructure is needed, like gas stations, electrical cables and roads. Thanks to economies of scale, that infrastructure also gets more efficient. This is not an unfamiliar idea; in a bigger city, people are closer together, they travel shorter distances, more municipal resources are shared. But what Bettencourt and West showed is that this increase in efficiency always has the same magnitude. A city with twice as many people as another has more infrastructure than the smaller city, but has about 15% less infrastructure per person. That figure is about the same, whether you’re looking at roads or pipelines, and whether you are looking at the difference between a city of 100,000 people vs. 200,000 people, or the difference between a city of 1 million people vs. 2 million people.

The bigger the city, the more productive (by 15%)

Some factors are the outcomes of interactions between people. These include both good outcomes – like innovation and wealth – and bad outcomes– like crime and disease. Because social interactions increase with population size, so do these outcomes. But as for infrastructure efficiency, these social factors not only increase with size but also become increasingly productive with size. That means both per person wealth and crime rates increase, but again with the same regular pattern: a doubling of the population results in an increase in a 15% increase in productivity per person.

West thinks these patterns are telling us something fundamental about the ways cities work – he is looking for a ‘unified theory of cities’ (ahh, physicists). I think we need to see more work from this group to decide how important these ‘laws’ are, but there is one thing they have convinced me of. We need to take these patterns into account when we compare cities. Traditionally, we look at measurements expressed per capita (divide the measurement by the number of people in the city). But while NYC has more crime per capita than Indianapolis, that is mostly a consequence of its size. What we really want to know is whether NYC has more crime per capita than predicted by its size. That is how we could tell whether some local factor – town planning, law enforcement, politics, history, geography – is playing an important role in crime rates.

Bettencourt and West have made a start on these kinds of comparisons. They looked at the stats for every city in the US corrected for population size. Some cities, like NYC and Indianapolis, are just about average for their size. Other cities are unusual for their size, suggesting the influence of local factors. Some aspects of a city seem to be more susceptible to the influence of local quirks than others. For example, the population size of any city predicts its gross metropolitan product (income) with about 93% confidence. However, you can only predict the annual number of patents arising from the same city with about 67% confidence. That means that in many cases, things other than population size are driving rates of technical innovation.

Like in Corvallis, Oregon. This city of 80,000 peopled filed 128% more patents than predicted from its size alone, making it the most disproportionately innovative city in the US. Corvallis, Oregon? That would be the home of Oregon State University and a large Hewlett-Packard operation that in its glory days developed the commercial inkjet printer. As an aside, it is also the least religious county in the nation, the one with the most Peace Corps volunteers, and the one with the most green buildings. You get the picture. Meanwhile, the average personal income in this creative city is only about 10% greater than predicted by its size. The question that now arises is whether the cause of Corvallis’ uniqueness is simply the Hewlett-Packard campus, or whether some other unique factors are at play. Corvallis would certainly hope it’s the latter, given the layoffs that have drastically reduced Hewlett-Packard’s local operations.

I first heard about this work during a talk by West at his academic home, the Santa Fe Institute, which brings together experts in modelling complex systems. The talk was being given at the fabulous Santa Fe Science Writing Workshop, and West’s audience consisted of of science writers and scientists hungry for a story for their writing assignment. After the talk, I was buzzing with excitement but surprised that the reception from the others was lukewarm. Everyone agreed the work was fascinating, West had given brilliant presentation, and they were now overflowing with stories. But they also agreed that the whole project was clearly mad. The chemist was skeptical of how squeaky clean the correlations were. The historian saw all the crucial details of history whitewashed out.

When I read accounts of this work in the press, critics mostly complained that West isn’t telling us anything we don’t know. Others pointed out that this work doesn’t tell us anything about what makes one city different from another.

But aside from suspicion of the actual data processing (which I can’t critique), I think these criticisms miss the point. Looking at common trends isn’t a way to deny the effects of history and politics – it’s a way to highlight them.  If you don’t believe me, you should spend some time playing with the interactive maps they generated from the US data. Get used to the interface by tracking the performance of your favourite city over time, then start to notice the profound influence of history in the patterns of red and blue dots. West’s approach just quantifies the things we might guess –the US is unequal, crime and patents and money are linked to human capital– and tells us where to start looking for informative exceptions.

Bettencourt LMA, Lobo J, Strumsky D, West GB (2010) Urban Scaling and Its Deviations: Revealing the Structure of Wealth, Innovation and Crime across Cities. PLoS ONE 5(11): e13541.

Bettencourt LMA, Lobo J,Helbing, D, Kühnert C, West GB (2007) Growth, innovation, scaling, and the pace of life in cities. PNAS  e13541. 104(17):7301-7306