Reference for Eruvin 5:4
אֵין מוֹדְדִין אֶלָּא בְחֶבֶל שֶׁל חֲמִשִּׁים אַמָּה, לֹא פָחוֹת וְלֹא יוֹתֵר. וְלֹא יִמְדּוֹד אֶלָּא כְנֶגֶד לִבּוֹ. הָיָה מוֹדֵד וְהִגִּיעַ לְגַיְא אוֹ לְגָדֵר, מַבְלִיעוֹ וְחוֹזֵר לְמִדָּתוֹ. הִגִּיעַ לְהָר, מַבְלִיעוֹ וְחוֹזֵר לְמִדָּתוֹ, וּבִלְבַד שֶׁלֹּא יֵצֵא חוּץ לַתְּחוּם. אִם אֵינוֹ יָכוֹל לְהַבְלִיעוֹ, בָּזוֹ אָמַר רַבִּי דוֹסְתַּאי בַּר רַבִּי יַנַּאי מִשּׁוּם רַבִּי מֵאִיר, שָׁמַעְתִּי שֶׁמְּקַדְּרִין בֶּהָרִים:
We measure [the two thousand cubits of the tchum Shabbath] only with a [flaxen] rope of fifty cubits, not less, [for when the rope is short, it is stretched more and the measure increases], and not more, [for when it is longer, its weight doubles it in the middle and it shortens.] And one measures only against his heart. [The sages fixed a place for the end of the rope, each against his heart. For if one placed it against his heart and the other against his feet, the rope would shorten and the tchumin would diminish.] If he were measuring and he came to a valley or a fence [a fallen stone wall that became a high, slanted heap], he "swallows it up" [If it were not fifty cubits wide from edge to edge above, even if its incline were more than a thousand, we do not say that it is included in the measurement of the tchum; but one stands on one side and one on the other, and the incline is swallowed up with one rope], and he returns to his measure. ["He returns to his measure" implies that if its width in the direction of the city were more than fifty, so that he could not swallow it up there with the rope, and with one of its ends not in the direction of the city, he could swallow it up — he goes and swallows it up there, and he keeps on walking and measuring from the edge onwards until the spot where the width of the valley ends in the direction of the city, and he continues measuring in the direction of the city and completes the measure of the tchum.] If he came to a mountain, he swallows it up [This, if the mountain is not very steep, but on an incline, so that walking five cubits of it lifts one only ten handbreadths; but if it is so steep that less than five cubits of walking lifts one ten handbreadths, he does not swallow it up, but only estimates (its distance) and goes on.], so long as he does not go outside the tchum. [When the measurer goes to "swallow up" the mountain or the valley, he may not go outside the tchum to a place where the heads of the valley are so narrow that he can swallow them up, in order to return thence to his measurement in the direction of the city — a decree by reason of the possibility of one's seeing him going and measuring there and saying that the tchum measurement of the sides of the city extends that far.] If he is unable to swallow it up, about this R. Dostai said: I heard that mountains are "bored through." [("about this R. Dostai said":) "about this," to exclude (the measurements for) cities of refuge and for the red heifer, (the city) closest to the slain man, where there is no boring through. ("bored through":) They (the mountains) are regarded as bored through, and they are measured through the hole to exclude the measurement of the slope, as stated in the gemara. It is measured with a four cubit rope. The lower (measurer) places the rope against his heart, and the higher, against his feet, and they measure the whole, four cubits after four cubits progressively, so that a four cubit slope loses half a man's height. The halachah is in accordance with R. Dostai.]