Sentence:  The mass scale used in Table 5 - 1 was derived on the <WSD>assumption</WSD> that the motion of the glowing trail is related to the momentum transfer to the trail by the meteorite , permitting the calculation of the mass if the velocity is known ( Cook and Whipple , 1958 ) . 
Meaning of the word marked with <WSD> tags:  a statement that is assumed to be true and from which a conclusion can be drawn