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In the last video we talked about how humans over time

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have been getting better and better, getting more Calories out of a given unit of land.

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And one thing I want to emphasize, in the last video

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these numbers that I came up with, I picked these numbers

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to make the map fairly simple and to give you a general idea,

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I don't want you to think that what a square kilometre

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can support today is exactly 1,000 people.

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It depends hugely on what the land is like,

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how much of the land you're actually using for agriculture,

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what crops you're planting etc. etc. ..

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how the people are living, how many Calories they need ...

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the whole point of the last video was just to give you a framework

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that, wow, there is this upper bound based on how much productivity you actually get from the land.

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Now, what we want to think about in this video

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that was kind of the last video with this axes right here

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gettin more and more out of the land,

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but what I want to think about in this video,

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is how did humans through different technologies,

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how do we get by doing less and less of the labout for getting those Calories out of the land.

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Obviously, you just don't have land and things spontaneously grow

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like I guess would happen in the wild,

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but if you're doing agriculture, you need to put some energy into the land,

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you've got to work the land.

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And so what we have over here in this chart,

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and this chart is derived from inforamtion from this book right over here,

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this is Energy And Society, and what we want to think about here

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is the different ways that humans have gone about to till soil.

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So not if you're going to think about the total process

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or the total energy required to grow a certain crop,

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what we're just going to focus is one step of the agricultureal process

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and that is tilling the soil.

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And, in case you're like me, and you have never worked on a farm,

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that's one thing that I would like to change one day, actually go through that process,

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but tilling the soil is kind of turnig it up

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so you get the nutrients from the bottom layers to the surface,

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you dig, you bury all of the remains from the last harvest,

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you bury all of the weeds so that they die

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and you also kind of get air in the soil

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what is does is it essentially prepares the soil for the next agricultural cycle.

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So it's a process that humans have been doing since antiquity.

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And what I want to do in this video is to think about the different ways to do it

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and how much energy is required to do it.

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And we're going to think about the enrgy in two ways,

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how much of that energy comes from humans,

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and how much of that energy comes from things other than humans.

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So just as an example. When we talk about human power,

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we're talking about someone literally hand-ploughing the field.

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So this woman right over here is literally, she has a sort of cart

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that's digging up the soil behind her.

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When we talk about the oxen power, we're talking about the oxen doing most of the work.

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They're the ones dragging this plough which is digging up all of the soil

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and this gentleman has to kind of be there to supervise.

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But this still is fairly an intense labour that this gentleman is doing right over here.

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And when we talk about the tractors, we're talking about a scenario like this,

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where the tractor is doing most of the work of actually digging up,

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dragging this plough behind it and digging up all of the soil.

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And from this book right over here, that's where we've got these numbers from,

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I'll tell you which numbers I've got from them

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and which numbers I kind of reason through

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because I wasn't fully comfortable with the numbers they had.

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But these are their numbers. That if your human power

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to till 1 hectar of soil will take 400 hours.

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Oxen power, this should be a pair, not pari, a pair of oxen, 65 hours.

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6-HP tractor 25 hours.

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50-HP tractor 4 hours.

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And in case you are wondering what is a hectare of soil,

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it is literally a plot of land, a hectare of soil,

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a hectare of land I should say,

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is a plot of land that is 100 m by 100 m

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and it's roughly equal to 2.5 acres, not exactly 2.5 acres,

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it's like 2.47... something, but roughly 2.5 acres.

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When you think about how many hours to essentially dig up all of the soil

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for a plot of land 100 by 100 metres.

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So what we have over here, so clearly human takes a lot longer,

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oxen make it a little bit faster,

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6-HP tractor even faster, 50-HP tractor, very powerful tractor, even faster than that.

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Now, this column right over here is the amount of energy

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required to actually produce and maintain the machinery used.

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So this is a very unintuitive thing. For example, whenever you think about

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the amount of energy to plough this land over here,

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you tend to think: ok, this individual is going to expend a lot of her energy,

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you don't think about the amount of energy required to actually maintain the tool,

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to build the tool that she is using, in this case a hand plough,

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and then to maintain that as she does it.

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And so this estimate, I've got these two from these fellows right over here,

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they're actually more and there might be a gal?,

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but it's about 6,000 kilocalories, and this is kilocalories for lower case c.

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And one thing I wanna emphasize here,

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1 kcal is equal to 1 Calorie with a capital C

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which is the same thing as 1,000 calories with a lower case c.

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We talked about this in the last video,

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but when people talk about food calories

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they are really talking about the Calorie with the capital C

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or you can say they talk about kilocalories.

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So your candy bar, 200 Calories, they are talking about this right over here,

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in Chemistry class you talk abou the amount of energy to rise a gramme of water 1°C,

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you're talking about these calories here.

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So all of these numbers in this chart right over here,

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you can either use them with this unit, kcal, kilocalories,

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or Calories with the capital C, they're essentialy the same units that we used in the last video,

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and these are the same numbers that you are used to from a kind of a dietary calorie point of view.

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So for example, 6,000 Calories, that's about how much a typical male would expand in 3 days,

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so this is to maintain it over the course of these 400 hours in the case of the hand plough,

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and the total amount of Calories that were needed to make the plough,

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devided by the total number of hours.

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So whatever fraction of the plough's live is being used here,

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you used that fraction right over here to put this 6,000 Calories.

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But needless to say, for at least the plough, for the kind of the ...

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either the human or the oxen scenario, this isn't a significant amount of the total Calories.

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So obviously, if you are doing either with human power or oxen power,

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you are not usin any gasoline, you are not using any petroleum,

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in all of these scenarios we're going to assume,

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that someone has 10 working hours in the day.

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And that right over here this is kind of a measure of how hard that person's work is.

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I kind of estimated these numbers here,

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they are slightly different than what the original numbers were in this book right over here.

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But they are saying: look, if you are actually walking along using this hand plough,

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that is actually very vigorous activity.

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It is going to require about 400 Calories per hour to do this type of activity,

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you do it over 10 hours, it's going to require 4,000 Calories

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just to do it over 10 hours, and then we are assuming that the rest of the day

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you are going to walk around, maybe you cook dinner, eath breakfast, sleep some of it,

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we are assuming the other 14 hours of the day are going to be about 100 Calories per hour,

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and so this is the total if someone were to using this techique

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work for a total of 10 hours, this is how many Calories they would consume in the day.

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You can see this is the most labour-intensive,

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so it looks like that they would consume the most Calories per day,

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these two are the least labour-intensive, you are sitting on the tractor,

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although that still requires more Calories than sleeping or watching TV,

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and so that is the number of Calories they would consume.

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Now this right over here, this is the interesting number, or one of the really interesting numbers,

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based on all of these assumptions, this is the total human input in Calories to do this task,

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to till this 1 hectare of soil.

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So over here, you are using 5,400 Calories a day,

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it you are working 10 hours per day and it requires 400 working hours,

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you are going to be working 40 days, right, 400 divided by 10,

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40 days times 5,400 Calories per day, it is going to take a human,

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just the human part, not even thinking about the 6,000 Calories necessary to maintain and make that plough,

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the human is going to expend 216,000 Calories to till, to plough that 1 hectare of land.

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And if you add the other 6,000 into for the actual plough,

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and you can debate what this number should be,

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because it is not a significant number compared to this,

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you get about 222,000 total Calories.

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When you go to the oxen situation requiring fewer hours

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and each hour requires fewer ... a little less Calories,

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this is still labour-intensive, but not as labour-intensive as what this women right over here is doing.

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So at daily base is she using a little bit fewer Calories,

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but since you are only doing 6.5 days of this, 65 hours divided by 10,

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you have significantly reduced the number of Calories, the total number of Calories

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that the human need to put into this task.

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Now, there still is other energy being done.

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Now all of a sudden the oxen have got involved.

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And if you assume that each ox consumes about 20,000 Calories a day,

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and you have 2 of them, so 40,000 Calories per day for just to feed the oxen,

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and you are going to do that for 6.5 days, 65 divided by 10,

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the oxen are going to consume 260,000 Calories to do this task.

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So the total energy input here now has gone up.

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So this is an interesting phenomenon that is going on right over here.

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What the human is putting in as we get better and better technology goes down substantially,

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216,000 and 33,000, and we will se that with the tractor that goes down even more,

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but the total energy, if you include the amount of energy that the oxen have to put in,

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or if you include the amount of energy to do the gasoline that has to be used for the tractor,

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the total amount of energy is going up to plough that field,

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but the human energy goes down dramatically.

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Now, the last thing I want to highlight here, and this is where my numbers

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that depart a little bit or fairly significantly from this original study right over here, this original estimate,

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is the machinery input on the tractors.

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So if you look this up, you can google search it, they have much larger numbers here,

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but I did a little research and it looks like for most petroleum, combustion based engines or vehicles

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roughly 20% of the total energy that is used in the field,

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20% of that energy is used for the actuall production and maintenance of that veihicle over its life.

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So what we did over here is we said: ok, for a 6-HP tractor, I used their number,

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we are going to have to use 25 hours to do it,

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it is going to use this much petroleum,

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assuming that is uses 23.5 litres of gas or petroleum over 24 hours,

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and I just put 20% for that number for saying

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how much energy had to be used to maintain that vehicle over that amount of time,

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and if you think about what fraction of this vehicle's life that 25 hours represents,

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that fraction times the total amount of energy required to produce that.

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So remember, these things are made out of metal, they had to be made in furnaces,

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so just pruducing a vehicle requires a lot of energy,

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and so this right over here is 20%, and I just used the rule of thumb,

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for most petroleum or combustion based vehicles, the 20% of the total energy expenditure

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over the course of that vehicle's life is roughly equal to the amount of energy used to produce that vehicle.

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But either way, you go all of the way over here, the human has to spend less Calories sitting on the vehicle,

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so they spend less Calories per day,

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and then the total human input right over here for the 6-HP tractor,

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it is going to take them 2.5 days, 25 hours, 10 hours a day,

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is going to be 8,500 Calories. But of course, you have the petroleum used,

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and then some estimate of the amount of energy used to produce that tractor,

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and you are just taking the fraction over that 25 hours,

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you are not taking the entire life of that 6-HP tractor.

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To produce this 6-HP tractor this number would be much larger if you talk about the total number of energy,

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we are just taking this small fraction of its life that we are using right over here,

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same thing for the 50-HP tractor.

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But anyway, if you look at it, this is the really interesting thing,

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humans, by going from human power all the way to a 50-HP tractor,

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you are getting almost a factor of 200 improvement

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in terms of how little energy has to be put in by the human to till that land.

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But you actually get a total increase if you factor in things like the petroleum,

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and then definitely the amount of energy to actually produce that machine.

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So anyway, hopefully you found that interesting.

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I find this kind of ... it's something that you don't think a lot about. How much energy input has to be put in.

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And often times we only think about the human energy input,

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but we are not thinking about the energy input from other things like oxen.