Making a Floating Lantern: What Should You Consider?

[wisdom32]

im trying to make a lantern float but what kind of formulas should i consider in trying to make it float? i want it to be like this. http://upload.wikimedia.org/wikipedia/commons/9/94/SkyLanternRichy01.jpg

 

[mgb_phys]

It simply has to weigh less than the volume of air it is displacing.
The weight and volume of the envelope are simple to measure.
The weight of the hot air inside will depend on the temperature, at room temp 24litres of air weighs around 29g it decreases linearly with absolute temperature ( ie deg C + 273)

You will have to experiment to work out how hot the air is on average - or find a site on make model hot air balloons