Introduction:
The blacksmith was able to notice the entry of the peddler in the forge due to various reasons. Let's delve into the details to understand why the blacksmith was able to perceive the peddler's presence.

1. Acoustic Awareness:
The forge is a place where the blacksmith spends most of his time working with hot metal and tools. As a result, he develops a heightened sense of auditory perception, allowing him to pick up even the faintest of sounds. Any footsteps or movements made by someone entering the forge would create distinct sounds that the blacksmith would easily recognize amidst the usual background noise of the forge.

2. Familiarity with Surroundings:
The blacksmith is intimately familiar with his workshop and its surroundings. He knows every nook and cranny, every sound, and every movement that typically occurs in the forge. Therefore, when an outsider like the peddler enters the premises, the blacksmith's familiarity with the environment allows him to quickly detect the presence of an unfamiliar individual.

3. Change in Atmosphere:
The entry of a person into a space often brings about subtle changes in the atmosphere. The blacksmith, being accustomed to the forge's ambiance, would be sensitive to such alterations. These changes could include variations in temperature, air movement, or even the scent of the forge. The blacksmith's perceptive nature enables him to pick up on these changes, alerting him to the entry of the peddler.

4. Visual Observations:
Although the forge is a place filled with smoke, fire, and glowing metal, the blacksmith's eyes are well-adapted to the environment. He can discern movement and detect even slight changes in his field of vision. Hence, if the peddler entered the forge, the blacksmith would likely catch a glimpse of the unfamiliar figure, prompting him to notice the peddler's presence.

Conclusion:
The blacksmith's ability to notice the entry of the peddler in the forge can be attributed to his acute auditory awareness, intimate familiarity with the surroundings, sensitivity to changes in atmosphere, and visual observations. These factors combined enable the blacksmith to quickly detect any external presence in his forge, ensuring a sense of security and awareness within his workplace.