 Now, let us see the effect of change, we will look at two scenarios now. One scenario is the airship is operating from some altitude to an altitude which is below the pressure altitude. Now, listen very carefully, you have to apply your mind and understand what is happening. So, let us say the pressure altitude is 1500 meters, you are operating from 500 meters to 600 meters. The airship changes altitude from 500 meters to 600 meters. So, I would like to know the net static lift will not change as long as you operate below the pressure altitude. Grass will change, grass static lift is going to what, decrease or increase? Grass static lift is going to decrease or increase, somebody is saying increase, somebody is saying decrease. Decrease or increase? Decrease. Decrease. No. Decrease. Decrease, fine. Why will it decrease? Because density of the air is decreasing with the altitude, so the grass static lift will decrease. Basically, grass static lift is what, volume and density difference. So, density difference, density of the gas inside is not changing, we are assuming the super pressure, super heat etc. We are assuming the density inside remains same. So, density outside is falling, so the difference is decreasing. Therefore, the grass static lift will decrease. However, as you go up, what do you do with the ballonet, you throw out some air. So, what is the increase in the volume of the lifting gas, it is equal to a decrease in the volume of the ballonet. Correct? So, the net static lift will not change as long as you maintain below pressure altitude. Because the rate of the ballonet air released is equal to the reduction in the grass static lift. Some amount of ballonet air is thrown out. By that amount, the lifting gas volume is increasing. So, as long as you maintain your altitude from any altitude up to pressure height, there is no change in the net static lift. But, there is change in the grass static lift and there is change in the ballonet air, volume and hence weight in the previous slide. So, under ISA conditions, the temperature that is prevailing at a particular altitude is equal to the temperature in the ISA table. And essentially, I have actually replaced the pressure also, PS is standard, TA is equal to TS and PS is anyway equal to PS. So, both the pressure and temperature are going to be as you have in the ISA table. That is how we got the ratio of pressure by temperature as equal to the ratio of pressure temperature at sea level and sigma. See, what we have done is we have used rho P by T is equal to rho P by T. So, at the pressure altitude, P is equal to P th, T is equal to T th, rho is equal to rho h, rho is equal to rho at p h and at sea level, it will be P 0, P 0 and sigma rho 0. So, when you take a ratio, you will get rho h by rho 0, which is sigma. So, P by T is equal to P by T at sea level into sigma. So, we have assumed both of them to change. So, this is clear. So, we can summarize that if delta operating, that is change in the operating altitude is below the pressure altitude, then delta ln of the net lift change is 0 since delta w ba is equal to delta lg. Now, this thing is not true when you go above the pressure altitude because above the pressure altitude, we are not throwing out the air, we are throwing out the gas and gas is lighter than air. So, to that amount, there will be lesser weight loss. So, there will be a change in the aesthetic lift. So, above pressure height, what are we doing? Lifting gas is released instead of ballooned air. Now, the density of the lifting gas has to be less than the density of the ambient air, that is why we are getting buoyancy. So, there is something called as relative density, it is always less than 1. And further you are from 1, better is the lifting gas, hydrogen is the best one, but helium is normally used, so it is 0.1382. So, the density of a given amount or given volume of helium is only 13.82% of the density of the given volume, right. So, therefore, if you throw out helium instead of air, you are not going to throw out heavy things, you are going to throw out lighter things. So, the weight of the lifting gas released is only Rd into Lg, RdLg times the gross static lift. So, therefore, the net static lift therefore will be reducing. So, the net static lift is going to reduce above pressure height, because the change in the lifting gas is going to be not equal to the change in the gross lift, but a small percentage of that. So, this is an important observation and which we should understand and keep in our minds.