Hi, P-san,
Let's go with it, OK?
Since you brought in a dictionary, then here we go!
This is one of the dictioneries that compare the two words.
Dictionary: The American Heritage® Dictionary of the English Language 4th edition 2000
silence
NOUN: 1. The condition or quality of being or keeping still and silent. 2. The absence of sound; stillness. 3. A period of time without speech or noise. 4. Refusal or failure to speak out.
silent
ADJECTIVE: 1. Marked by absence of noise or sound; still. 2. Not inclined to speak; not talkative. 3. Unable to speak. 4. Refraining from speech: Do be silent. 5. Not voiced or expressed; unspoken: a silent curse; silent consent. 6. Inactive; quiescent: a silent volcano. 7. Linguistics Having no phonetic value; unpronounced: the silent b in subtle. 8. Having no spoken dialogue and usually no soundtrack. Used of a film. 9. Producing no detectable signs or symptoms: a silent heart attack.
You see, the basic difference is whether "speech"part plays in the meaning or not. Of course you can see, "Silent" is used just like "Silence" per #1 definition of Silent. you can see that, right?
So, as you can see as in example#6 and your example of engine (a noun form of "silent") indicate both are things naturally "talkative", known to make sound.
Thus, if you change your example of "The silentness and smoothness of the new engine in this car will surprise you" to "The SILENCE and smoothness of the new engine in this car will surpise you"、 now the new sentense sounds <g> strange. Isn't it?
I hope you can see my point made here.
The same token, a sentense "The silence of the morning calms me to appreciate my life in the peaceful age" will sounds strange if it is changed to "The SILENTNESS of the morning calms me to appreciate my life in the peaceful age". But it sounds just fine if the sentense is now changed to "The SILENTNESS of the morning calms me to appreciate my life surviving in this war age".
The last sentense indicates the "FEELING" of "every morning is very noisy because of the sound of the bombs and the tanks. But this morning is different, "the moment of silentness" is really appreciative."
Please note the #1 defination of silence. The key word is "KEEP". So it indicates it is "normally silent"
This is another reason why we say "break the silence", not "break the silentness", because the silentness might make the readers understand that it is known as to be not silent all the time.
With an exagerated expression (meaning, most likely we would not say or even think), "breaking the silentness" may mean "go back to the noisy situation". Meanwhile "Breaking the silence" simply indicates the silence is over, or disturbed.
But, this difference usually does not play a big role, unless it (the difference) is needed to be expressed, such as in a novel or a poem. But I hope you can see how these words are used differently, not just as a noun and an adjuctive but there is "hidden" as we call it "silent" meaning. <g>
Mr. Phaedrus, Thank you very much for another opportunity to express my OWN understanding of the feeling of English words,though might very well be an isolated usage. And please do "feel" my sincere admiration to not only your knowledge but also yours ways to explain in proper and easy-to-understand Japanese. I sure hope my writings have not made you upset.
And also I want to wake sure that everybody understands that my way of explaning or my own feeling toward English words may not be fully correct, but that simply they are from my life experience "using English" here in US.
Ciao and cat'ya later, my friends.
お礼
Ganbatteruyoさん、ご回答どうもありがとうございます! アメリカに35年もお住まいということで、心づよい です☆ fallは(~の状態に)なる、陥るという、becomeと同じ意味 として考えればいいということなんですね。allowの意味に ついては、「許す」として考えていたのでいまいちよく 分からなかったのですが、そういう意味もあるんですね。 勉強になりますー。。ご親切に本当にどうもありがとうございました!