Sentence:  If we could get the Post Office Department to adopt the metric system, so that whenever any citizen of the United States desires to send a letter he will have to buy postage stamps of one or two cents — whatever it might be — for 30 grams of matter, the <WSD>metric ounce</WSD>, that would give us a start. 
Meaning of the word marked with <WSD> tags:  30 grams.