[functions]

My chemistry text 'Chemistry The central Science' by Brown wrote:

Although scientists of the nineteenth century knew nothing about subatomic
particles, they were aware that atoms of different elements have different masses.
They found, for example, that each 100.0 g of water contains 11.1 g of hydrogen
and 88.9 g of oxygen. Thus, water contains 88.9/11.1 = 8 times as much
oxygen, by mass, as hydrogen. Once scientists understood that water contains
two hydrogen atoms for each oxygen atom, they concluded that an oxygen
atom must have 2 X 8 = 16 times as much mass as a hydrogen atom. Hydrogen,
the lightest atom, was arbitrarily assigned a relative mass of 1 (no units).
Atomic masses of other elements were at first determined relative to this value.
Thus, oxygen was assigned an atomic mass of 16.

My question are
1)What is mean by 'relative mass' as in 'Hydrogen, the lightest atom, was arbitrarily assigned a relative mass of 1 (no units)' and why there is no any units?
2)How is the atomic masses of other elements were determined relative to the arbitrarily assigned relative mass of an hydrogen atom?

[opti384]

It just works like this. A hydrogen atom's mass will be really really small. So the chemists just assumed the hydrogen atom's mass as 1 (without a unit). An oxygen atom's mass was about 16 times greater than that of a hydrogen atom. Therefore, the relative mass of oxygen will be 16.

[Fluoroantimonicacid]

Actually,they consider ₁H's atomic mass 1.008.

[Borek]

Actually,they consider ₁H's atomic mass 1.008.

That's because hydrogen mass in now listed relative to C-12. When H was assumed to be 1, atomic mass of the carbon-12 was 11.90.