«Thus can my love excuse the slow offence
Of my dull bearer when from thee I speed».
The companion to the previous sonnet, Sonnet 51 further expands on the theme of traveling.
Thus can my love excuse the slow offence
Many of the details in Sonnet 50 appear here, including the “slow offence / Of my dull bearer,” which mirrors “The beast . . . / Plods dully on,” and the relative weight of different emotions: the heavy weight of sadness in the previous sonnet compared to the light, effervescent weight of desire in Sonnet 51.
Sonnet 51 mixes the present with the future — what the poet termed “onward” in the last line of the previous sonnet. The first four lines occur in the present, but line 4’s “Till I return” prompts the poet to think about the future. However, unlike the grievous future in Sonnet 50, this future is joyful, for the poet believes that his thoughts of love for the young man will accelerate his return: “Then can no horse with my desire keep pace.” Note that this desire is characterized as “fiery,” which recalls Sonnet 45, in which the poet imagined desire as a “purging fire.”
English audio from YouTube Channel Socratica
Summary from Cliffsnotes.com