  Let $n, k$ and $a$ be positive integers. The Stirling numbers of the first
kind, denoted by $s(n,k)$, count the number of permutations of $n$ elements
with $k$ disjoint cycles. Let $p$ be a prime. In recent years, Lengyel, Komatsu
and Young, Leonetti and Sanna, Adelberg, Hong and Qiu made some progress in the
study of the $p$-adic valuations of $s(n,k)$. In this paper, by using
Washington's congruence on the generalized harmonic number and the $n$-th
Bernoulli number $B_n$ and the properties of $m$-th Stirling numbers of the
first kind obtained recently by the authors, we arrive at an exact expression
or a lower bound of $v_p(s(ap, k))$ with $a$ and $k$ being integers such that
$1\le a\le p-1$ and $1\le k\le ap$. This infers that for any regular prime
$p\ge 7$ and for arbitrary integers $a$ and $k$ with $5\le a\le p-1$ and
$a-2\le k\le ap-1$, one has $v_p(H(ap-1,k))<-\frac{\log{(ap-1)}}{2\log p}$ with
$H(ap-1, k)$ being the $k$-th elementary symmetric function of $1, \frac{1}{2},
..., \frac{1}{ap-1}$. This gives a partial support to a conjecture of Leonetti
and Sanna raised in 2017. We also present results on $v_p(s(ap^n,ap^n-k))$ from
which one can derive that under certain condition, for any prime $p\ge 5$, any
odd number $k\ge 3$ and any sufficiently large integer $n$, if $(a,p)=1$, then
$v_p(s(ap^{n+1},ap^{n+1}-))=v_p(s(ap^n,ap^n-k))+2$. It confirms partially
Lengyel's conjecture proposed in 2015.
